The purchase of an aircraft isn’t as simple as kicking the tires and taking a test flight.  Most aircraft have a significant dollar value to the buyer and seller.  The value of the airplane is directly affected by the sum total of the state of repair of all of the airframe, powerplant, and a myriad of installed equipment.  To verify the value of the airplane, dozens of “good-bad” assessments of the operating condition of the plane must be made.  These assessments are based upon not only the physical inspection of major components but also the condition of the supporting accessory components.

Buried in the powerplant package are the magnetos.  A simple observation may be to run the engine, flip the mag to left and right to see what the RPM drop is and how the engines run on the individual magnetos.  If the magnetos are working, they are working, if the engine runs, it runs.  Right?

Well, maybe not so simple.  While the magnetos may pass a basic engine run-up inspection, there are lots of not so obvious logbook records for required maintenance which keeps the magnetos in good operating condition.  New airplane owners may get hit with “catch up maintenance” to the tune of thousands of dollars if the inspection and Airworthiness Directive compliance of the magnetos require attention.

It is a certainty that various maintenance issues will be identified during the inspection.  Repairs will either need to be done or inspections may be due at some point after the purchase of the plane.  These issues are not likely to substantially affect the purchase price of the airplane unless particularly expensive or of negative impact on the operation of the aircraft or engine. 

Ultimately, the prepurchase inspection benefits the buyer to know what maintenance requirements are upcoming for the purposes of budgeting and scheduling maintenance.  If money needs to be spent after buying an airplane to correct maintenance issues, better to know this upfront.  As the saying goes “better to be advised than surprised!”


The very first step is to confirm that the magnetos installed on the engine match the logbook entries.  No surprise, but logbook records can be messy and entries to record installation and removal of magnetos may be incomplete.  

A common scenario is that a magneto was installed in a hurry and the work was never recorded in the logs.  The result is that the serial numbers of the installed magnetos actions of Airworthiness Directive compliance and 500-hour inspection in the logbook will not match the unrecorded, but installed, magneto.  Verify the installed part number and serial number and then review the logs to match to required or recorded maintenance events.

Regarding maintenance events, magnetos will have some fundamental inspection points which should be recorded in the logbook:

  1. Airworthiness Directive Compliance
  2. Service Bulletin Compliance
  3. Calendar Time Overhaul 
  4. 500 Hour Inspection


The logbooks may have a list of Airworthiness Directive AND Service Bulletin compliance, but is the list complete and accurate?   The ADs and Service Bulletins on Bendix and Slick mags are too numerous to list in this discussion and should be researched for current effectiveness (Kelly Aero ES magnetos do not have any Airworthiness Directives, so a much shorter discussion!).  

Surprisingly, a complete listing for ADs for both Slick and Bendix may not be found by searching the FAA or magneto manufacturer databases.  Some ADs are only found when searching on the engine model, and in some cases, the magneto AD is against the airframe (!!).  Of note, one Slick Airworthiness Directive, AD 88-25-04, is applied against the airframe and requires instrument panel placards and Airframe POH amendments.  A great example is that the deep dive into records may extend beyond the logbooks.

The bottom line with logbook AD records:  Trust but Verify.  It is likely that the previous mechanics have done the required work.  But, any mechanic working on an airplane and engine new to them will need to research and review that the magnetos match the records.


All magneto manufacturers require an overhaul of magnetos based on calendar time, regardless of hours.  The idea is that a magneto that is not operated frequently is perhaps even more likely to experience service issues than a magneto operating 1,000 hours in one year.  

Bendix has a detailed Service Bulletin SB643C that details how some Bendix magnetos are subject to either a 5-year overhaul or 12-year overhaul, depending upon the serial number and model number specifics.  Both Slick and Kelly detail 12-year overhaul requirements in their manuals.  

The basic idea here is that, when purchasing an airplane with a low-time engine, the date when the engine was overhauled, may have some unforeseen implications.  The magnetos may have low operating hours.   But, the low operating hours’ overtime may trigger manufacturer requirements for inspections that could potentially incur a substantial expense for the new owner.   


The record of the 500-hour inspection may be the single most important record in the logbook.  The 500-hour inspection is a routine maintenance event that can be performed at any time during the service history of the magneto.  This inspection can be used to remedy Airworthiness Directive compliance and other inspection requirements if the magneto has an unclear service record. If the logbook shows no record of 500-hour inspections and the magnetos have more than 500 hours, then the inspection is due.

Consideration at the prepurchase inspection is to negotiate with the seller to complete the magneto inspection as part of the sale. Or, perhaps split the cost?  It is a relatively low-cost inspection, but the benefit is that both the seller and buyer are assured that the magnetos are back to a known baseline.  For the seller and buyer, the Kelly Aero 500-hour inspection provides a warranty for the magnetos and simply removes any post-sale liability concerns for the ignition system after the aircraft purchase.

If the seller is not willing to provide the 500-hour inspection, then the buyer should consider spending the money to get the inspection done.  Once again, the benefit is that the new owner can have Kelly Aero baseline the magnetos to a known condition.  With one less thing to worry about, the new owner can fly their new airplane with greater confidence that unplanned magneto maintenance will not keep them grounded.

Have Fun and Fly!  Harry Fenton, Director of Business Development and Product Support

Do you have any blog suggestions or want to know about Kelly Aero products?  Send us a note and we will answer your question:

Retro Aircraft Air Conditioning

Retro Aircraft Air Conditioning

Getting to take out your aircraft on a  warm summer’s day can be a highlight of a good weekend or the best kind of start to your week. Taxiing out onto the tarmac to get ready to fly has to be one of the best feelings of anticipation there is. But sitting there sweltering in the heat coming off the hot asphalt can really put a damper on the excitement, and leave you just wishing you could be back inside rather than getting to enjoy the hard work you’ve put into making this plane ready for flight. 

Luckily, Kelly Aerospace has the ability to help you get air conditioning into almost any older remodeled plane you bring to us. We are able to kit our FAA certified air conditioning systems to fit in most Piper, Cessna, Beechcraft 

Our thermal cooling systems are lightweight, with most models coming in at under 60 pounds when fully installed. They work using a hermetically-sealed, brushless DC compressor. In most older aircraft, the system is installed in the back behind the usable space and is therefore convenient and out of the way leaving full use of the baggage area. Check out this video of the system being installed in this beautifully restored Beechcraft Baron. Each aircraft that comes to us is specially measured and fitted by our engineering team, and they modify our kit as needed to fit into your aircraft. We build our own components such as hoses, evaporators, and connectors in house, and thus are easily able to adjust as needed. 

Our cooling systems offer pre-cooling on the ground, meaning that from the moment you step into your plane to the moment you take off the ground you can be enjoying sitting in the cockpit and getting ready for your flight. We generally recommend starting up your cooling system before you plan to hop in your plane, as with most aircraft it takes approximately 5 minutes to experience a 20-degree temperature change, especially if your plane has been sitting on a hot runway or it is an especially hot day out. 

Our cooling systems all feature an easy-to-read digital temperature and fan control, with the amperage required to run the system ranging from 40-60 amps. Our systems are able to be used pre- during- and post-flight with no restrictions on the engine or impact on cruising speeds. 



By Harry Fenton, Director of Business Development and Product Support

“Timing” is equally a simple and extremely complicated term relative to the installation and operation of magnetos.  However, the word “timing” as applied to magnetos are a set of terms that means one thing, and many things, depending upon the context of the terms:  Internal Timing, External Timing, and Advance Timing are all examples of individual timing terms.  The sum total of all of these terms becomes what is broadly referred to as Magneto Timing.

These terms are basic knowledge to aircraft technicians and educated owners.  However, Kelly Aero Product Support sometimes finds that not all Kelly Aero customers completely understand the specific terms or the interaction of these terms with one another.  A basic understanding of the basics is critical to the maintenance and ongoing safe operation of the aircraft ignition system.  So, let’s review!


Magneto timing to the engine is directly based on the “time” of when and where the piston is located during its travel within the cylinder relative to the combustion chamber.  Piston travel is accomplished via crankshaft rotation of all of the connected engine parts to push the piston.  

While seemingly an instant explosion, the combustion occurs over a period of time relative to the travel of the piston within the cylinder.  The combustion process begins before the piston is at the point of maximum travel within the cylinder and continues to burn until the piston reaches its peak point of travel.  At the peak point of piston travel, the fuel is completely burned up, and thermal energy is released resulting in high combustion pressures in the cylinder.  These combustion pressures push against the piston to drive the crankshaft in a forward direction to turn the propeller.

For the combustion process to begin the magneto must produce a high energy spark at the exact right moment of time relative to piston travel.  To do this, the “Internal” and “External” timing of the magneto must be exactly correct.  The magneto must be “timed” to deliver a spark so that the process of combustion, which occurs over “time”, can occur as the piston travels within the cylinder.


Internal timing is the alignment of the various moving internal components of the magneto to ensure that the spark generated by the magneto is of the maximum intensity and discharged at the correct magneto to engine timing position to ignite the fuel mixture in the combustion chamber.  


When installed on an engine, the magneto is physically connected, or “timed” to the rotation of the engine crankshaft.  The magneto alignment to the engine synchronizes the spark generated by the magneto to be delivered to the combustion chamber at the correct moment of the combustion cycle of a particular cylinder.  


Top Dead Center, or TDC, is the furthest position of piston travel within the combustion chamber to what is considered the “top” of the combustion chamber.  The timing position of TDC is referred to in degrees of rotation from this reference point.  TDC is at 0 degrees of travel, neither advanced nor delayed, but at the peak point of crankshaft rotation.  


The ignition of engine combustion occurs in a crankshaft and piston position before TDC at a position called Before Top Dead Center, or BTDC.  BTDC is a bit of a tongue twister, so it is most commonly referred to as the “Advance” timing position.  Advance is always referred to in degrees and is not always the same from engine to engine.


After Top Dead Center, ATDC, is a timing point after TDC, and is considered to be a delayed timing position.  In technical terms, this timing position is referred to as a “Retarded” timing point.  While an odd term is simply a technical description that the spark is delayed beyond TDC relative to crankshaft rotation.


BDC has no useful purpose for ignition timing.  All magneto-based ignitions used on any for or six-cylinder aircraft engine will never need to be timed at BDC. If a magneto is timed to fire at BDC, then this is an immediate concern and the timing of the magneto to the engine must be corrected.


Magneto timing is set to deliver a spark at the Advanced BTDC timing piston so that the engine can make maximum horsepower at full throttle.  This advanced timing position is an extreme negative to start the engine, however.  At low RPM, during start, the piston is advanced to crankshaft rotation.  Any spark or combustion that occurs at this point will tend to force the piston and crankshaft backward in rotation.  The end result is hard starting, and, most likely, damage to the starter.

If the spark is delayed, or retarded, to occur near the TDC position of the piston, the combustion will drive the piston and crankshaft in a forward direction.  The magneto or magnetos used to start the engine must be fitted with special parts that are used only during the engine starting mode.  The two options are a mechanical device called an impulse coupling, or the second set of contact points that are connected to a relay that electrically boosts the magneto during engine start.

In either configuration, these devices delay or retard the timing of the spark to initiate combustion.   The degrees of delay between the Advance and Start position is termed “Lag Angle”.  


Over the course of upcoming Kelly tech talks, the terms discussed today will be applied to all of the parts that make a magneto work.  Keep watching for the new discussions!

Do you have any blog suggestions or want to know about Kelly Aero products?  Send us a note and we will answer your question:

MAGNETO HARNESS NUT SIZE – 5/8 or 3/4? What’s the difference?

MAGNETO HARNESS NUT SIZE – 5/8 or 3/4? What’s the difference?

By Harry Fenton, Director of Business Development and Product Support

When a customer orders a Kelly Aero ignition harness one of the choices is to order a harness with 5/8” or 3/4” nuts.  Why are these seemingly unrelated numbers used to describe an ignition harness?

These fractions represent the dimensions of the internal diameter and thread pitch of the nuts that attach the ignition lead to the spark plug.  These dimensions have no reference to the width of the nut between the “flats” of the nut.

The 5/8-24 nut is 5/8” internal diameter with a 24 thread per inch count, or “pitch” in technical terms.  The 3/4-20 nut, likewise, is 3/4” in diameter with a 20 thread per inch thread count.   A more common reference is simply “small” (5/8”) or “big” (3/4”) for the nuts.  The spark plugs, in a similar description, are referred to as “small barrel” (5/8”) and “big barrel” (3/4”).

Spark plug development began with unshielded spark plugs that had a simple clip-on connector.  The downside to these spark plugs is that the connection was open to the elements.  Water or mist could easily cause the spark plug to ground out, malfunction leading to partial engine failure.  Another issue was that the spark plugs were often exposed directly to airflow around the engine and the simple clip-on connector could shake loose and detach from the spark plug.  Finally, the unshielded spark plugs were totally unsuitable for radio communications as all of the electrical noise produced by the spark plugs were not controlled by an effective shielded connection.

Airline, corporate and military aircraft development in the 1930s led to the development of “shielded” spark plugs.  Shielded spark plugs are defined as the primary ignition lead connections to the spark plug:  A threaded nut tightly secures the ignition lead nut to the spark plug.  Additionally, the contact hardware of the ignition lead terminal was inserted into a protected chamber inside of the spark plug, outside of the elements.  The protected ignition lead solved the problems of weather and vibration compromising the operational reliability of the spark plug lead.

Post-WWII, the aviation spark plug settled into the two distinct types available today, commonly referred to as the 5/8” Small Barrel and 3/4” Big Barrel.  Of note, the 3/4” spark plug is referred to formally as an “All-Weather” spark plug.  The All-Weather Spark Plug is a shielded spark plug specifically designed for high-altitude operation. The ignition lead insulator is recessed into the shell to allow a rubber grommet on the ignition harness to provide a watertight seal.

As a rule, most normally aspirated, parallel valve cylinder Lycoming engines are typically built at the factory with 5/8” spark plugs.  Most Lycoming angle valve, and all turbocharged angle valve engines are typically fitted with 3/4” All-Weather spark plugs.  Most Continental engines, both 4 and 6 cylinders, built prior to about 1980 used the small barrel 5/8” spark plugs.  All current production Continental engines are now fitted with 3/4” All-Weather spark plugs as standard equipment.


Before ordering a harness, identify the spark plug part number installed in the engine.  All spark plugs with REB, REL, REJ, or REM in the part number are 5/8”.  All spark plugs with RHM and RHB in the part number are 3/4”.  Visual identification of the installed spark plug is best as the part number is stamped on the spark plug shell.  Be cautious of invoices or logbook entries with spark plug part numbers as it is not unusual for the aircraft records do not match the installed parts.  

5/8” and 3/4” spark plugs can also be identified by the distinct features of how the ignition lead nut mounting threads are cut into the shell of the spark plug:

The single easiest way to identify ignition lead nuts is to use a wrench as a gauge. The spark plug socket size is not useful to identify the spark plug type.  All currently manufactured aviation spark plugs use the same size socket for the main hex on the plug body:  7/8”.  But, the wrench that fits a 5/8” nut is a 3/4” and the wrench that fits a 3/4” nut is a 7/8”.  Remember, the nuts are not based on wrench size, but on the internal threads and diameter of the nut.  This chart is a handy summary of the wrenches used and their corresponding nut sizes.


The single most common mistake is that an owner buys a 3/4 harness instead of a 5/8.  Why?  As detailed previously, a 3/4” wrench fits the 5/8 nut. So…must be a 3/4 harness nut, right?  Unfortunately, no. 

An important step BEFORE installing the ignition harness is to confirm that the spark plugs are the correct match.  But, all too often, customers fully install an ignition harness only to find after all the work that the wrong harness was ordered and the nuts won’t fit the spark plugs.  The problem becomes that once the harness has been installed on the engine, it is considered used.  While returns and exchanges are not impossible, it can be a difficult process after the harness is installed.

Can the ignition lead hardware be changed to match the spark plugs?  The blunt, honest answer is that it is impractical and not cost-effective to change all of the ignition lead hardware.  The process is time-consuming and requires special tools, manuals, and general experience with ignition lead repair.  While repairing a single lead makes sense, changing all the hardware on 8 or 12 leads is just not practical.

There is a solution, which is simple and much less expensive than converting the hardware on a spark plug lead:  Buy new spark plugs!  Superficially, this may sound crazy, but run the math on labor and parts to convert the ignition leads versus a set of new spark plugs.  For the most part, new spark plugs will be much less expensive.  The good news is that virtually all engines approved for 5/8 spark plugs are also approved to use the 3/4 spark plugs, so converting from one spark plug style to another is usually not a problem. However, consult manufacturer data to confirm spark plug applicability for your engine.

Do you have any blog suggestions or want to know about Kelly Aero products?  Send us a note and we will answer your question:

Magneto Noise Filter?

Magneto Noise Filter?

By Harry Fenton, Business Development and Product Support

Lots of older Bendix magnetos are returned to Kelly Aero either as cores or for a custom overhaul with large external capacitors attached to the outside of the magneto.  Much to the surprise of the customer, this big capacitor is not replaced or provided as new as part of a Kelly Aero overhauled magneto.  In fact, this capacitor is not shown in the magneto overhaul manual or parts list, and there is little to no documentation as to how these parts are installed with the magneto.  So…what is this thing?

This big capacitor is called a “Radio Noise Filter”  and its intended purpose is to reduce radio noise generated by the magneto.  It is a relic of a bygone era as an attempt to fix radio static problems experienced in aircraft radios used in general aviation airplanes of the 1950s and 1960s.  While not useful for current aircraft and avionics, these capacitors are still sold by Kelly Aero as a spare part.


When the magneto is operating, the contact points close to allow the magneto to build a charge, and then open to allow the current to discharge which creates the ignition spark.  The result is a pulsating voltage produced at the internal magneto capacitor, and this voltage is carried by the P-Lead and switch.  

For the most part, this voltage has no effect on the engine or magneto and is simply a byproduct of normal operation.  However, the pulsating voltage in the P-Lead can result in a low amplitude radiated signal that has the potential to create a signal that can be picked up by radio.  

Because this signal is not a structured signal in the form of voice or morse code to identify VOR stations, it is termed “noise” and it is interpreted as interference.  Radio noise is a rare occurrence and is as described:  A steady whine, a general static or a staccato clicking sound as the spark plugs fire that is heard while tuned into aviation radio frequencies.

A lot has changed in the world of avionics from the 1950s to now.  One of the serious problems in the 1950s and 1960s as radios became more common to the airplane cockpit, was “radio noise”.   Early radios worked on dozen frequencies for voice and navigation and the radio designs were based on frequency crystals and vacuum tubes. The old radios were very susceptible to radio noise due to the limits of the components and circuit designs available at the time.  On top of that, all of the components like lights, strobes, generators, spark plugs, ignition harnesses, and magnetos generated some sort of radio noise.  From the radio standpoint, it was a noisy environment.  

Noise filters, in the form of capacitors, were attached to all sorts of wires connected to lights, generators, and magnetos.  The idea was that the filter would simply change or diminish the radio frequency of the noise generated by these devices to something outside of the aircraft radio frequency range.

Modern avionics manufactured since the late 1980s onward are digital, microprocessor controlled with much better filtering and much greater capability to reject noise.  Additionally, FAA certification required that airborne emitters of electrical noise- like magneto ignition systems- keep the level of radiated electrical noise within acceptable levels.


For the most part, airplanes and engines built from the 1980s onward eliminated the noise filters as an extra part that was subject to maintenance or failure just wasn’t needed.  Radio noise was cured by better shielding of airframe wires, lower noise emissions of magnetos, ignition wire, and spark plugs.  The noise filters just were no longer required as they were Band-Aids to other issues such as unshielded P-Leads.  A subtle issue is that the extra noise filter is in-line with the P-Lead and just another component subject to failure that could have a negative effect on magneto operation.  In short, just another part that is subject to failure, so if it is not needed, it can easily be removed from the ignition system for greater overall system reliability.


Having said that, Kelly Aero still sells a lot of the MF3A Ignition Noise Filters.  Why?  We are not exactly sure, but there is a continuing demand for these parts.  One reason may be that A&P mechanics are conservative and want to reinstall the replacement magneto to match, in every exact detail, the magneto that was removed.  If the magneto was originally fitted with a noise filter, then a new Kelly Aero MF3A noise filter is installed to replace the old, worn-out noise filter.  Admittedly, the noise filter may serve a purpose on some older airframes or experimental airplanes where the wire shielding and ground paths are not as well designed towards reducing the effects of radio noise.  Mechanics and amateur aircraft builders can use the filter as a tool to isolate the root cause of radio noise interference.

P-Lead connections are likely the most practical reason why the noise filter continues on in service.  Most of the older Bendix magnetos used “bayonet connectors” for P-Leads.  The bayonet connector is configured with a large nut, and an insulator, looking much like the spark plug connector on an ignition wire.  The airframe P-Lead is configured with a #8 ring terminal.  If the filter is discarded, then airframe P-Lead needs to be re-configured from the ring terminal to the bayonet-style hardware.  The easiest path is the just re-use the old noise filter on the replacement magneto.  This is discouraged by Kelly Aero as, in effect, an untested, decades-old part of unknown history is attached to a freshly overhauled magneto.  There is a lot of risk to degrading the service life of the magneto with the worn-out and unneeded noise filter.

But, if an installer wants to install a noise filter, then the Kelly Aero MF3A is the best option.  The current Kelly Aero MF3A noise filter is configured with a ring terminal on the wire that connects to the magneto.  This connection works with all current manufacture Bendix short cover 20/200 Series, Bendix 1200 Series, and all Slick 4300/6300 and Kelly ES4300/6300 Series magnetos.  But, long cover Bendix 20/200 Series magnetos will need the ring terminal on the MF3A to be replaced with bayonet hardware as pictured below.

One final note:  Remove the noise filter before returning the core magneto back to the parts supplier or back to Kelly Aero.  While the filter is not required, it is useful to have to get the replacement magneto quickly installed until the airframe P-Leads can be reconfigured or a new manufacture noise filter like the Kelly Aero MF3A is installed. 



By Harry Fenton, Director of Business Development and Product Support, Kelly Aero

Today’s world is dominated by modern, high-tech smart electronics that can be found in every device
imaginable from toothbrushes to spacecraft operating billions of miles away from Earth. General
Aviation airplanes are equipped with the latest glass panel, GPS driven avionics that have more
computing capability than any manned space vehicle that was sent to the moon. Aviation has
historically been on the cutting edge of the newest and best technology found in the cockpit, so the
expectation is that there should be an equally new technology applied to the aircraft engine and its
systems. But, cutting-edge engine technology has been stubbornly slow to change piston aircraft

In particular, why is it that mechanical magnetos- which have been in use on reciprocating engines for
over 125 years- are still being used as the primary ignition systems for piston-engine aircraft? With all of
the modern technology at our fingertips, why isn’t there something better? Auto engines have not used
contact points for a few generations. It is likely that the parents of the high school students learning to
drive today never drove or owned a car with an engine using a mechanical ignition system. Mention
Magneto to these generations and the only reference they will have is a Marvel comic book character.
Yet, magnetos remain the most prevalent ignition system used for aircraft engines. If asked, most
aviation enthusiasts believe that aircraft engines use magnetos because that is the only ignition system
approved by FAA Regulations. It is true that the Civil Aeronautics Authority, the predecessor to the FAA,
defined the standards for piston-engine aircraft ignition systems nearly 85 years ago. The wording in the
current regulations has remained virtually unchanged since then, and reads as follows:
33.37 Ignition System
Each spark-ignition engine must have a dual ignition system with at least two spark plugs for each
cylinder and two separate electric circuits with separate sources of electrical energy, or have an ignition
system of equivalent in-flight reliability.
Interesting….where are the words that say, specifically, magnetos must be used on piston-engine aircraft
engines? The truth is there is no specific guidelines set forth by the FAA that piston engines must use
magnetos as the primary ignition source for piston engines.
So, why are magnetos used as the most prevalent ignition system used on aircraft piston engines when
more modern technology for ignition is available?


The discussion of aircraft magnetos needs to begin with some sort of historical context as to how
magnetos were first designed onto aircraft engines. As with virtually all the basics of flight, magnetos
can be directly traced back to the engine used in the very first Wright Flyer of 1903.
Everyone knows that the Wright Brothers built and flew the first successful powered airplane. But, very
few people know that the Wrights also built the very first piston engine specifically designed for
airplanes. The engine used by the Wrights was one of the most important, but overlooked elements
that made their airplane successful.

The Wrights were not the first to fly or developing sophisticated aircraft designs. George Cayley, Otto
Lilienthal, Octave Chanute had flown manned gliders many years ahead of the Wrights and had proven
the concept of heavier than air flight. Samuel Langley made successful flights with an unmanned
airplane powered by a steam engine. Langley unsuccessfully launched a steam engine-powered, man-carrying airplane in October 1903, 3 months prior to the Wright’s historic flight.
The important point is, numerous inventors had developed flight-capable airframes at the time that the
Wrights were experimenting with flight. However, the airframes lacked a suitable powerplant of the
right power and weight that could propel the airplane in powered flight. The engine used by the
Wrights solved the propulsion problem and directly contributed to their accomplishment to
demonstrate controllable, powered flight of a heavier than air machine.
The Wrights were assisted in their aircraft engine development work by their in-house master bicycle
mechanic, Charlie Taylor. Keep in mind that gasoline-fueled piston engines were an emerging science of
the time, and not common at all. The vast majority of people living at the time had never seen nor
heard a piston engine and very likely had no knowledge of how a piston engine worked. In 1903, when
the Wrights completed their first powered flight, Henry Ford was still 5 years away from producing the
very first Model T car, so gasoline-powered engines used in vehicles were a rarity.
Incredibly, with no formal engineering background, using only the skills he had learned as a toolmaker
and bicycle repair mechanic, Charlie Taylor built the first successful airplane engine in only six weeks!
He did follow established engineering concepts for piston engines of the time and used design elements
from existing, successful engines. For the ignition system, he used what all other engine manufacturers
were using- a magneto!
What inspired Charles Taylor to use a magneto? Was there a better solution to be found in the automotive
world? In a word- No. In 1903, the magneto was state of the art for ignition systems, was the very best
solution for a lightweight, simple, self-contained generator of electrical energy. The magneto did not
require any external sources of power to make it generate spark energy. The engine flywheel turned a
magnetic rotor shaft in the magneto, and an electrical charge was generated. That energy fired the
spark plug to ignite engine combustion which made the engine run to turn the propellers.
The only other option available to Taylor was a battery ignition system that supplied power to an
external coil and contact point mechanism to distribute the spark. The dilemma for Taylor was that
batteries and generators of the time were extremely heavy, with all-up weight in the many hundreds of
pounds. The size of the batteries also would have required considerable physical space, extra structure-
and resulting weight- to support the batteries in the Flyer.
The magneto solution used by Taylor was an engineering marvel. The average magneto weight was 20-
25lbs. and made a spark any time that the engine was running. The empty weight of the Wright Flyer
was just over 600 lbs., meaning the magneto system was just under 5% of the total weight of the
airplane. A battery ignition system probably would have added 30% more weight to the Wright Flyer,
which would have clearly prevented it from flying.
Is it a stretch to suggest that the Wrights were successful as the first to fly a controllable airplane due to
the magneto? While that is an interesting idea, it is safe to say that the magneto certainly contributed
to the overall success of the Wright Flyer and the Taylor engine.


The current day Federal Aviation Administration, or FAA, can find trace its roots through a number of
government agencies that were focused on defining the regulations for aviation safety. The Civil
Aeronautics Authority of the 1930s put into effect more stringent regulations to improve the safety of
aircraft and engines. The early Civil Aviation Regulations became the later Federal Aviation Regulations
and established the basis for rulemaking and safety standards for aircraft and engine design.
Through the 1920s and early 1930s, the aircraft engine continued to rely upon single magnetos as the
primary ignition system. As the CAR’s developed to improve upon aircraft engine design and safety, the
Regulations for a dual, independent ignition system made the spark generating magnetos a perfect
solution to comply with this government requirement. Dual ignition systems became the standard
design, and for good reason. If one ignition system malfunctioned, then the remaining ignition could
keep the engine running so that the flight could continue and be landed normally, under complete

The magneto also made sense as it was uncommon for aircraft of the time to be fitted with electrical
systems or starters. Aircraft electrical systems did not develop as quickly as they did for cars, primarily
due to weight, complexity, and expense. Batteries, starters, and alternators were still very heavy and
not particularly reliable. The added weight of an aircraft electrical system could easily add 200 lbs. to
the aircraft weight or about the weight of a passenger and personal baggage.
For the most part, pilots of the time did not care that there were no electrical systems on
airplanes. With no electric starters, aircraft engines used the “Armstrong Method” to get engines
started: The propeller was swung by a person using their arms, the mags were switched to on, and the
engine started. No worry about dead batteries, no worry about the cost of maintaining starters and
electrical systems, no worry about getting stranded due to a failed electrical system. If the pilot could
swing the propeller, the magneto sparked, and the engine would run. Magnetos provided the perfect
solution to provide simplified system installation, good starting characteristics, and low cost of


In the early 30s, aircraft ignitions and automotive ignitions took different paths in terms of
development. Automakers favored battery-driven contact point/coil/distributor systems and aircraft
engines remained steadfast with the magneto. There are numerous technical reasons why each system
worked better in some way for either the automotive or aircraft application.
First, the mission profile of automotive engines and airplane engines became distinctly different. Auto
engines are subject to a frequent change of RPM to speed up and slow down, sometimes driving in stop-and-go traffic, sometimes driving fast for long distances. Because of this, auto engine ignition systems
and timing to the engine were biased to improve starting, idle, and low to mid RPM acceleration.
Magnetos are limited to “fixed” advance ignition timing for all operations other than starting the engine.
The fixed advanced timing works for airplane engines because full power is required at takeoff, and
engine RPM does not vary for all inflight operations after takeoff and landing. The requirement for the
aircraft engine to make full power is critical to flying safety. Airplanes must carry their certified load at
takeoff. Not only that, but they must takeoff within a specific length of the runway and climb at a rate
sufficient to clear obstacles or terrain within the vicinity of the airport.
Auto engines rarely need to run at full power for extended periods, and rarely run at greater than 30%
to 40% power most of the time. Car drivers never worry about having enough power to clear a hill or
the power required to drive with light or heavy loads. Full engine power is rarely if ever, required for a
typical passenger car. Because auto engine RPM varies when driving and the engine accelerates or
decelerates randomly, the fixed timing of the magneto does not provide the best overall performance.
Magnetos were not optimum and automakers devised distributors with “variable advance” mechanisms
that changed timing based on the centrifugal force applied to the advance mechanism as the engine
RPM changed.

The advanced mechanisms had the potential for failure modes which could affect ignition reliability,
though. The advance mechanism itself adds many extra components to the system, all of which are
subject to maintenance, or in the worst case, failure. Auto ignitions were designed to default to an
engine timing point not at full power, but to low power, idle timing position. The automotive failsafe
provided for “limp home” capability, but at the cost of reduced power.
The default timing for the aircraft engine magneto is the maximum power timing point, which provides
for the safest situation should the engine be required to continue running after one of the ignition
systems fails in flight. The lack of the advanced mechanism is a benefit in terms of maintenance and
overall cost due to lower parts count.
The picture comes into focus that aircraft engines and automotive engines have distinctly different
“mission profiles” relative to how the engine develops power relative to ignition timing.
 Automotive engines timing is designed to provide the best starting spark and optimize spark at
less than full power engine loads by varying engine timing
o The limp home default in the event of a component malfunction is for reduced engine

 Aircraft engines timing is optimized to perform best at full power engine loads by keeping
ignition system timing at a fixed point with no variability
o The limp home default is normal operation provided by the remaining ignition system
should one system fail


The FAA’s single-minded goal for safety does not necessarily inhibit innovation, but it can encourage
aircraft engine manufacturers to follow conservative, simple design paths of engineering. However, is a
conservative path, wrong, or just as pragmatic as a mindset of “not re-inventing the wheel?”
There have been at least a half dozen electronic ignitions specifically designed to replace magnetos
introduced into the aviation market since 1986. But, none of these ignitions have shown the potential to
be the “perfect” replacement for magnetos. Incredibly, many of the electronic ignitions sold today
require that a magneto be retained as part of the system for failsafe backup. When the electronic
ignition fails, the old technology, tried and true magneto will save the day so that the aircraft engine
continues to run safely

In the final analysis, electronic ignitions are challenged to match the simplicity of installation and repair
support that exists for magnetos. By design, electronic ignitions are more complicated installations with
numerous components and wiring connections that all have to be not only installed correctly but
maintained correctly. Troubleshooting of electronic ignitions requires the ability to think in more
abstract terms of electronics and component interactions. Magnetos are mechanical, maintenance and
troubleshooting do not require any extraordinary troubleshooting skills. The vast majority of aircraft
mechanics in the world know how to install, maintain and repair magnetos. Out of the hundreds of
thousands of aircraft mechanics in the world, only a few hundred may have experience with installing
maintaining, and servicing aircraft electronic ignitions.
Parts and service support for magnetos are unparalleled. Magnetos, parts, and companies that can
service magnetos can be found worldwide. Anywhere in the world where a piston engine airplane can
take off or land, there are magnetos parts or support available within a one-day shipping time. In most
cases, maintenance shops based at airports with higher levels of airplane activity will have parts in stock
and mechanics available immediately to provide service for the magneto. Due to the very low
population of electronic ignitions, virtually no repair parts are easily found in the worldwide market.
Parts are stocked at a handful of locations, or available as a special order from the manufacturer. In some
cases, electronic ignitions sold and installed at some point in the past are simply no longer supported by
the manufacturer. The recommendation from the manufacturer is to replace that electronic ignition
with magnetos should it need service!
So why is it that mechanical magnetos- which have been in use on reciprocating engines for over 125
years- are still being used as the primary ignition systems for piston-engine aircraft? When all of the
advantages and disadvantages are summed up, the very reason that Charlie Taylor selected the
magneto for the first piston aircraft engine remains as true today as it was 125 years ago: The magneto is
a self-contained generator of electricity and ultimately the least complicated, most common sense
the choice for reliability and performance for aircraft engines.

OEM Maintenance Compliance & Kelly Aero Magneto Data Plate

OEM Maintenance Compliance & Kelly Aero Magneto Data Plate

By Harry Fenton, Director of Business Development and Product Support, Kelly Aero
Routine maintenance requirements and all special service actions required by Service Bulletins and
Airworthiness Directives typically require that the magneto serial number or part number be referenced
to verify compliance. The OEM data found on the magneto data plates is always the baseline to establish
the applicability of the compliance of a required maintenance or safety action.
However, what is required when an OEM data plate has been replaced by a company that is not the OEM
for the magneto? What if a serial number is added to the data plate that is different than the OEM serial
number? Do OEM requirements still apply? The bigger question: Is the magneto considered an FAA
legal part if the OEM data plate has been removed and replaced by a non-OEM data plate?
Replacement Data Plates- Is This Legal?
The simple answer is “yes”, Kelly Aero replaces worn and damaged OEM magneto data plates with new
Kelly Aero data plates. But, the “yes” answer is not as simple as it sounds. The FAA considers the
management and disposition of OEM data plates attached to FAA-PMA articles as serious business. The
real world for Kelly Aero is that there is no option: worn data plates must be replaced when magnetos
are inspected or overhauled. In most cases, magnetos returned to Kelly as cores for overhaul, or for the
500-hour inspection, may be legible, but not in a useable condition to continue on in service.

The continued use of the OEM data plate for ongoing service and compliance becomes a safety issue if
the information on the data plate becomes illegible. The only practical and safe solution is to make a
new data plate stamped with the information from the old, worn data plate.
The FAA has strict regulations against the removal and replacement of airframe, engine, propeller, and
life-limited parts, though. They spell out what can, and cannot be done in the Federal Code of
Regulations, Part 45. The FAA states, specifically, that data plates of Type Certificated or Life Limited
items may be removed in the course of maintenance and must be reattached to the item from which
they were removed. Replacement data plates for these items can only be provided by the OEM, but the
FAA must approve and accept that process. This concept has been drilled into mechanics thinking by
the FAA and industry guidance. The general understanding is that all data plates are forbidden to be

The data plates of component articles that are FAA-PMA approved, such as magnetos, are not as strictly
controlled and the FAA can approve a repair process that does not require OEM approval for data plate
replacement. Kelly Aero’s process to replace the worn OEM data plate with a new Kelly Aero data plate

is part of our FAA-approved Repair Station Quality Manual. This manual details the process to replace
data plates, but also documents the strict record-keeping and traceability procedure required to
preserve the OEM data plate and magneto model information.
Bendix and Slick magnetos worked on by Kelly Aero for overhaul or 500-hour inspection are completely
disassembled and all parts are cleaned and inspected to make the magneto look and work like new.
During the teardown and inspection process, the OEM data plate is removed from the magneto frame in
order to strip the paint from the magneto and repaint it to a new condition. The OEM serial number and
magneto part number are permanently coded onto the magneto frame to preserve the record of that
part. All of the information on the data plate becomes part of the extensive overhaul record of the
magneto as it progresses through the Kelly Quality system.
When the inspection and re-work steps are completed, a list of parts to overhaul or to complete the 500
hour inspection of the magneto is generated and added to the magneto record. At every step in the
process, from disassembly to completion of the overhaul, the OEM data is part of the documentation

After the magneto assembly is completed, a new Kelly Aero data plate is made for the magneto. The
data plate will show the OEM serial number, the Kelly Aero Overhaul serial number, and the part
number of the magneto. The new Kelly Aero data plate is attached and the inspected magneto looks as good
as new. An FAA Form 8130-3 Authorized Release Certificate is generated to document and release the
500-hour inspection magneto to service.
If the magneto is overhauled, the data plate marking will be slightly different as compared to the 500
hour inspection. Kelly Aero Overhauled magnetos are treated more like new production magnetos and
a unique Kelly Aero serial number is assigned to the magneto. The OEM serial number is also retained
and both the Kelly Aero and OEM serial numbers are engraved on the new magneto data plate.

The Dual Identity of a magneto overhauled by Kelly Aero
After overhaul, an FAA Form 8130-3 Authorized Release Certificate is generated to document
completion of the overhaul process and to authorize the release to service of this part by Kelly Aero. It is
at this point that the magneto develops a dual identity based on the specific requirements that apply to
the OEM serial number or the Kelly assigned a serial number. Both serial numbers become equally
important in terms of ongoing maintenance compliance.
The most common misconception is that the Kelly Aero data plate changes the requirements to comply
with OEM maintenance guidance. This perception is 100% wrong. All OEM Service Bulletins and
Airworthiness Directives continue to apply to the magneto based on the underlying OEM magneto data,
regardless if it was overhauled by Kelly Aero or by any other company.
However, the Kelly Aero overhaul uses Kelly manufactured FAA-PMA parts and additional processes of
inspection and workmanship which are not part of the basic OEM minimum requirements. These
additional features make the Kelly Aero overhaul unique. These unique features may be affected by
inspections or service needs different than, or in addition to, than OEM requirements. To track service
requirements of the content added by Kelly to an overhauled magneto, a unique Kelly Aero assigned
serial number is added to the data plate and is listed on the FAA Form 8130-3 Authorized Release
Certificate supplied with the magneto.

Given the “dual Identity” of the magnetos serviced by Kelly Aero, the replacement Kelly Aero data plate
is clearly more than just a simple cosmetic replacement. The Kelly Aero data plate ensures legibility for
the service cycle of the magneto, which is required for ongoing compliance and safety requirements.
The Kelly Aero data plate also adds the extra layer of traceability of the Kelly added components and
workmanship. Ultimately, ALL applicable OEM and Kelly Aero service requirements must be complied
with after the magneto is released to service, and the new Kelly Aero data plate assures that action can
be accomplished.
Do you have a question about Kelly Aero products or piston engine ignition systems? Contact us at

Magneto Installation Guide – Kelly Aero ES Series

Magneto Installation Guide – Kelly Aero ES Series


  1. Confirm that the part number of magneto is approved for the engine installation. Refer to the most current application data at
  2. BEFORE INSTALLING DRIVE GEARS OR ADAPTERS: Connect a magneto timing tool to the magneto P-Lead and confirm that the contact points open and close by turning the rotor shaft.
  3. As required, install magneto drive gears or adapters in accordance with engine manufacturer data. Torque rotor shaft nut to 120 to 320 in. lbs. Install cotter pin and secure.
  4. Identify Magneto Rotation
  5. Identify ES118T Timing Pin Features
  6. Verify that ES118T Timing Pin has been inserted correctly

Insert ES118T Timing Pin into hole that corresponds to magneto ROTATION noted on magneto data plate (ROT.)

NOTE: DO NOT insert Timing Pin based on position of magneto installation on engine

Timing Pin “Step” MUST seat against distributor block

ES118T Timing Pin MUST insert completely through distributor block and distributor gear

WRONG: Timing pin “step is NOT seated against distributor block

CORRECT: Timing pin “step” is seated against distributor block

Page | 1


Timing Pin Inserted Correctly LEFT ROTATION MAGNETO

  1. Follow engine manufacturer instructions to set up engine to advance timing position on cylinder number 1.
  2. Place gasket between magneto and engine and install magneto onto engine. Install magneto mounting clamps loosely to hold magneto in place but allow for movement of magneto to time to engine.
  3. Remove Timing Pin from magneto when magneto when magneto is loosely installed on engine.
  4. Adjust contact points to open at engine timing position. Tighten magneto mounting clamp nuts to 190 to 220 in. lbs.
  5. Connect ignition P-Leads to magneto capacitor and ground magnetos using the ignition switch.
  6. Install ignition harness to magneto.
  7. Test run engine in accordance with engine manufacturer instructions.
  8. Make appropriate logbook entries to document magneto installation.
Magneto Timing Synchronizer

Magneto Timing Synchronizer

Despite its simplicity, the magneto timing synchronizer, commonly referred to as the magneto timing tool, can be one of the most frustrating tools for even the most experienced mechanic to use.   The difficulty of using this tool can be baffling as the theory of operation is as simple as it gets. Connect a lead to the magneto ground point, connect a lead to the p-lead terminal, and as the magneto rotor shaft is turned, a light on the front of the tool will turn on and off as the contact points open and close. 

All too frequently, however, a mechanic will struggle with timing a magneto, unable to get the tool to indicate that the contact points of one or both of the magnetos are opening.  The typical action is to send the magneto back to Kelly for warranty inspection, only to be informed that the magneto contact points operated perfectly normally when checked on the bench.  

This discussion will take a closer look at how the magneto timing light tool can contribute to false diagnosis of a magneto problem.

The Tool

For the purposes of this topic, the Magneto Timing Synchronizer will be referred to as a Magneto Timing Tool.  Of course, this tool has other names, ranging Buzz Box as a nod to the buzzing or whistling sound that backs up the on and off illumination of the timing lights, to other, very salty terms when struggling with the tool on a late Friday afternoon.

From the theory of operation standpoint, the magneto timing tool is not a continuity tester, at least in the conventional sense of how a continuity tester works.  The principle of operation for a continuity tester is to introduce a voltage signal to a circuit.  If the circuit is open, there will be no continuity and if the circuit is closed or complete, then there is continuity.  If used on a magneto, the circuit is open when the contact points are closed, and the circuit is open when the contact points are open.

However, the slight introduction of a voltage is a theoretical safety issue as the voltage from a battery-operated continuity light or a multimeter can charge the electrical circuit and potentially enable it to discharge a spark.  So, at some point in the past, a method to passively detect changes to the magnetic circuit of the magneto, or inductance of the magneto electrical circuit, was established to be the correct method.  As such, all proper magneto timing tools are based on measuring induction, not continuity.

Inductive magneto timing tools are available in two different styles:  mechanical contactor type or solid-state type.  Both tools work to the same method of using lights and sound to signal contact point opening and closing, changing the indicating lights and sound based on changes to the magnetic density of the electrical circuit of the magneto.  

The gold standard of the mechanical contactor type of magneto timing tool has been the Eastern Electronics E50.  Hundreds of thousands of these tools have been produced, and it is virtually impossible to not find this tool in a well-equipped shop.  The theory of operation is that the contactors are energized by the internal battery, and when connected to the magneto to detect the contact points are open, the indicator lights will turn ON, and the tone of the tool buzzer changes.  The Eastern E50 does not have any instructions on the tool to indicate whether the indicator lights should be on or off when the points open, so the mechanic using the tool must confirm how the lights and buzzer actuate by grounding connecting the contact lead to ground to observe how the tool operates.

The solid-state magneto timing tool works in a similar way, except with no mechanical parts.  The sensing of the magneto flux is accomplished strictly by electronics, no mechanical contactors are used.  A big difference, though, is the lights turn OFF when the contact points open.  The obvious initial concern is that the mechanic using either tool MUST know how it works.  The solid-state units have instructions printed on the front that the lights will turn off, or be out when the contact points open.

The timing tool requires specific connections

Countless magnetos are incorrectly determined to be faulty due to the failure of the installer to connect the tool to the magneto correctly.  Follow these simple connection rules and the magneto tool will work as required to time the magneto.

Slick magnetos:  Connect the tool and use the fiber washer to prevent the tool lead from grounding and causing a false indication on the timing tool that the contact points are grounded and not opening.

Bendix magnetos

The short cover magnetos that use a simple capacitor stud for the P-lead connection require the same fiber washer to ensure that the timing tool lead does not ground and send the wrong signal.

Inspect the magneto with the timing tool BEFORE installation

Step One:  Confirm that the tool works before removing and installing magnetos.  Low battery voltage, especially with the solid-state tool, can still illuminate the lights, but is likely to not be sufficient to provide enough voltage to sense the change of the magnetic circuit in the magneto.  If the tool has not been used for several months, it is a sure bet that the batteries are weak and need to be replaced for accurate operation.

Step Two:  Check the magneto for operation BEFORE installing on the engine!!  With the magneto on the bench, connect the timing light and confirm that the timing lights illuminate correctly to show contact point opening or closing.  In addition, the internal timing of the magneto can very easily be confirmed before installing on the engine.

Slick Magnetos

  1. Insert the timing pin into the distributor gear in the hole that corresponds to magneto ROTATION, not the position on the engine.  For example, the right position magneto on a Continental O-200 is LEFT rotation, so the timing pin is inserted into the L hole.
  1. With the pin inserted, the rotor shaft can be moved very slightly.  The timing light should turn on and off as the contact points open and close.  If the pin has to be removed and the rotor shaft turned 90 degrees so the contact points open and close, then the internal timing is possibly incorrect.  The magneto should not be installed until the internal timing is confirmed or corrected.
  1. If the magneto passes the bench test, then it is ready to install.

Bendix Magnetos- 20/200/1200/Dual Magneto Series

  1. Remove the vent plugs on the magneto to expose the red painted gear tooth.
  2. Turn the rotor shaft so that the red painted gear tooth moves within the range of the vent plug hole.  The timing light should turn on and off as the contact points open and close.  If the rotor shaft is turned 90 degrees so the contact points open and close, and the red gear tooth is not visible in the vent plug hole, then the internal timing is possibly incorrect. The magneto should not be installed until the internal timing is confirmed or corrected.
  1. If the magneto passes the bench test, then it is ready to install.

Final magneto installation and timing

Making a solid ground lead connection between the magnetos and the timing tool is critical.  This is the single most often missed step when using timing light tools, and will invariably result in a false diagnosis of magneto timing or incorrect magneto to engine timing.

The magneto timing tool uses a very low voltage to power to power the tool to sense the changes to the magneto magnetic circuit.  The ground path from the tool to the magneto has to be direct between the magneto and tool, and as short as possible.  If the timing tool ground lead is connected to the engine, it is almost impossible for the tool circuit to complete the necessary ground path through the magneto to properly illuminate lights when the contact points open and close.

The best method to ensure a continuous ground path between BOTH magnetos and the timing tool is to connect an extra jumper lead.  Connect the timing tool ground lead to the ground point on the primary magneto.  Next, connect a jumper lead to run from the common connection of the timing tool ground on the primary magneto, to the ground point near the secondary magneto contact points.  This extra lead provides for a solid ground path and will eliminate the majority of timing problems in which the contact points seem to not open or close as expected.

DO NOT CONNECT THE TIMING TOOL GROUND LEAD TO THE ENGINE OR AIRFRAME!  The path to ground may not be connected if there is too much distance between the timing tool ground and the magneto ground.   

That’s it for timing tool discussion, give the techniques discussed a try.  If you have time, experiment with the tool to see where things can go wrong due to a mistake with a simple connection.  As always, feel free to suggest a magneto topic for future discussions.

Everything about Aircraft Ignition Harnesses

Everything about Aircraft Ignition Harnesses

One of the most common inquiries we receive at Kelly Aero regards ignition harness applications.  The basic engine application data is usually easy as most mechanics or owners will know that their airplane has a Lycoming or Continental engine installed, and perhaps they know whether Bendix or Slick magnetos are installed.  However, that sort of general information is only partially useful.  It is the very specific data on the configuration of the engine and magnetos that defines the correct features of the ignition harness.

So, what information or questions do you need to ask to determine the correct ignition harness for your engine?


#1  What is the engine model?

Invariably, the very first question asked by Kelly Aero Customer Service person is “What is the engine model?”  Some customers may start the conversation with “I have a 4 cylinder Lycoming” or maybe they know just part of the engine model or airframe, such as “Lycoming O-320” or “a Continental 550 in a Cirrus”.  The issue is that the very specific prefix, engine family and suffix will define the exact harness.  

Continental tends to use simple one-letter designators for their engines:  O-470-R, IO-550-N, for example.  Lycoming uses a system with more numbers in the suffix:  O-360-A4A, TIO-360-C1C6D which are good examples.  However, the harness fit, lead length, and sometimes special features such as seals for pressurized magneto applications will be different.  

The engine model can be found in the engine logbooks or in the Pilot Operating Handbook for the aircraft.  The information is also on the engine data plate, but that is only visible when the engine is uncowled.  

#2  What is the model number of magneto installed on the engine?

This is another item that requires some specific information.  Many customers will say they need a new harness for their “…mags..” with no other details.  The current production magnetos are generically referred to as Bendix or Slick, despite the fact that both the Bendix and Slick product lines have been sold by numerous companies.  But, Bendix and Slick are the standards, but there are variations within the basic magnetos.

The generic Bendix magneto models fall into the following categories:

  • 20/25/200 Series:  Most commonly, all of the compact size Bendix magnetos are referred to as the “20 Series” even though there are variations within the basic 20 Series of the 25 and 200 models.  The 20 series are offered in 4 and 6 cylinder versions, but this will be defined by the engine model as the 360 Series Lycoming is always four cylinder and an IO-520 Continental will always be six cylinder, and so on.
  • 1200 Series:  the 1200 Series is identified by its large size, and offered in 4 and 6 cylinder versions.  The difference between the 20 Series and 1200 Series is that the distributor blocks are much different in size and the ignition harnesses are not interchangeable. 
  • Slick 4300/6300 Series:  The Slick magnetos have more rounded features, and a data plate riveted to the side of the magneto, as opposed to the Bendix method of affixing the data plate to the top of the magneto.  The Slick harness cap is perfectly round and held to the magneto with three screws.  Typically, the Slick ignition harnesses will have a label that denotes the left or right harness part number, and that can be useful in determining the Kelly harness part number.

#3  What size are the spark plugs- 5/8” or 3/4”?  Or maybe asked as Small Barrel or Big Barrel plugs?

Historically, this detail trips up most private owners who are trying to buy parts for their airplanes.  The spark plug nuts must match the type of spark plug and mistakes made at the time of order will invariably result in buying the incorrect harness.

There are two types of spark plugs: SMALL barrel 5/8-24 or BIG barrel 3/4-20.  The numbers refer to the diameter and thread pitch at the top of the spark plug:

Aircraft Ignition Harness Spark Plugs, Small and Large Barrel threads.

Another easy way to determine spark plug nut size is to use a wrench on the flats of the plug.  BUT, be careful, as the correct wrench size can be interpreted for the wrong spark plug and harness nut size.  A 3/4” wrench does not remove a 3/4-20 nut!  Use the illustration below for guidance, but a 3/4” wrench is used to remove a 5/8-24 nut and a 7/8” wrench is used to remove a 3/4-20 nut.

#4  Straight or Elbow leads?

Virtually all modern engine installations use the straight lead, identified by a wire captured in a simple assembly just the small ferrule nut and the larger spark plug nut.  A formed tube that guides the wire into the spark plug nut supports the elbow lead.  

Straight leads are the industry standard, but Kelly offers the elbow lead for owners who prefer the extra support that the elbow provides.  Some aircraft, especially radial engine airplanes and helicopters, prefer to use the elbow harness for extra protection in the slipstream.  Current production airplanes with enclosed cowlings make the elbow redundant as the wire is protected within the cowling.  

#5 Time to Order

Now that all of the information for engine, magnetos, and spark plug size is known, the Kelly Ignition Harness Application Chart can be used to find the correct harness.  On the Internet, navigate to and click on the Support tab, and then Application Chart 

The Ignition Harness Application Chart will open and looks like this:

The chart is easy to use, the engine OEM and Models are listed on the left, and magnetos and spark plug sizes are listed along the top.  The letter “X’ denotes the column with the applicable magneto and the two far-right columns show the Kelly Ignition Harness part number.  As a lookup tip, most of the Kelly harnesses use part numbers similar to the Slick part numbers.  Kelly replaces the Slick “M” prefix with the “KA1” prefix.  In the example above, KA12772 is a replacement for the M2772 ignition harness.

Here are some examples of what it looks like to find common ignition harnesses:

Lycoming IO-360-A1A, Bendix 20 Series magnetos, 5/8”-24 spark plugs:  Harness is a Kelly KA12364, or a KA2364E if the elbows are required.

#6  Half Ignition Harnesses

Sometimes customers will require just a Left Hand or Right Hand side of the ignition harness, just for one magneto.  Bendix and Slick use unique part numbers for the Complete, Left Hand and Right Hand harnesses, Kelly Aero uses a simplified system of simply adding an “LH” or “RH” to the basic part number of the harness.  A very important note to identify Left and Right:  All Left and Right references are from the pilot’s perspective, looking forward through the propeller.  The wrong way to determine left and right is to reference left and right from the propeller, looking back at the tail of the airplane.

Here is an example:  A Lycoming O-360-A4A, Slick magneto installed on the left position, 5/8”-24 spark plugs.  The complete ignition harness is a KA14004, so to order just the Left side, order a KA14004LH.

That is it for the Ignition Harness selection discussion.  If you have any questions, call Kelly Sales, Product Support, or send an e-mail to us through the website link: