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 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

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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 Failure Overview

Magneto Failure Overview

Magneto technology is some of the oldest and most trusted in the aviation industry!
However, what happens when a pilot has magneto failure? When one magneto fails, there are a few things a pilot might notice. First, there will be a decrease in engine power, but the engine will remain running thanks to the second system being independent of the first. Second, sometimes a magneto can malfunction in its failure to ignite the spark plugs, or the internal timing may malfunction. In this instance, the pilot will notice the engine beginning to run rough and will need to switch to the remaining magneto and see if that clears the problem. 
Even though an engine will continue to run with the loss of one magneto, there are several problems with having a magneto failure mid-flight. With the loss of one magneto comes the security of redundancy. They are a vital piece of equipment, and there is a reason that aircraft are built with two. Additionally, the loss of engine power can be potentially problematic, though in most cases does not pose a serious threat to the safety of pilots and guests. 


Even in the case of magneto failure, only a very few accidents are a direct result of magneto failure, according to statistics provided by the National Transportation Safety Board. NRSB reports 16 recorded cases of aviation accidents due to magneto failure, with two of those sixteen being fatal. Most of these failures were listed as being caused by a lack of maintenance or non-compliance with servicing and inspections. 

With attention to detail and a history of reliability, Kelly Aero has made sure that the magnetos produced are dependable and safe. Our products are designed and engineered using in-house resources, with additional tooling and manufacturing for aftermarket magnetos occurring at our facilities. We offer an FAA and PMA approved quality system, which is readily available with low wait times. We both meet and exceed the standard industry testing with our own set of criteria for the product, whether it be the full system or a replacement part. We want to work with customers to ensure the delivery of a product that is first and foremost high quality and reliable, and that meets their needs.

Quality Upgrades at Kelly Aero

Quality Upgrades at Kelly Aero

One of the great additions to our processes in 2020 is a high power AmScope Microscope to our final inspection process.
This allows every our inspection team to get take a high power look at our finished units before they leave our facility.

Our new 80 power microscope allows us to inspect our block components for carbon tracking, carbon particles can collect in small cracks and develop into problems down the road.

“The Kelly Aerospace Quality Team is committed to adding new technologies to our final inspection process,” says Neil Clark (VP Sales & Marketing) “This addition allows us to go one step further in catching minor imperfections before they leave our facility.”

Doubling down our commitment to quality, this addition allows for a 99.9% rating on all blocks that leaving our facility free of imperfections.
Our engineering and development team has made major strides over the last few years to ensure that our processes at Kelly Aerospace continue to improve.

500hr Magneto Inspection/Overhaul

500hr Magneto Inspection/Overhaul

So your prized aircraft is down for an annual and your A&P says it’s time for your 500hr inspection.
Some pilots choose to overlook the importance of the inspection.
However, the 500-hour inspection is the single most important magneto service event.

Whether you are an owner, flight school, commercial operator, or engine overhaul shop, you can ensure continuing trouble-free and safe service of an aircraft magneto.

The 500-hour magneto inspection is critical to the safe operation of magnetos. While not a required service action as an airworthiness directive, it is considered critical and
mandatory by magneto OEM’s and is considered a good, common-sense maintenance practice by aircraft mechanics.

The 500 hour is scheduled preventative maintenance and to avoid expensive unscheduled maintenance. If a magneto is not inspected every 500 hours, then the risk of an unplanned component failure that can cause inconvenient and unplanned maintenance at a location far from an aircraft owner’s home airport is certain to occur.
The cost of one or more nights of hotel, car rental, meals, missed work, lost flight revenue will easily be offset by the relatively small cost of a 500-hour inspection.
The inspection is not limited to magnetos which have accumulated 500 hours in service. A 500
hour inspection can be accomplished at any hours of magneto total time in service. In some cases, some magneto and engine combinations may result in the need to perform magneto maintenance more frequently than 500 hours. Pressurized magnetos used on Continental and Lycoming engines operate in demanding, high altitude environments and require more maintenance than other types of magnetos.
Dual contact, or retard breaker magnetos that use the Shower of Sparks starting system typically require more frequent maintenance. Magnetos used in Shower of Spark applications require frequent
inspections to ensure that fuel-injected Lycoming and Continental engines do not experience hard
starting problems.

To begin the 500-hour inspection process, an inspection return form is downloaded from the Kelly Aerospace website. Complete one form for each magneto that is returned. Before boxing the magneto, remove the drive gears attached to the magneto. Return the complete magneto, with the drive gears removed, and do not forget to include the completed 500-hour inspection form. The magneto is returned directly to Kelly Aerospace, 1400 East South Blvd, Montgomery, AL 36116 with Attn
500-hour inspection marked on the box.
When the magneto is received, work starts within 24 hours of receipt. Plan for four working days to complete the inspection, plus shipping time to return to the customer. Kelly Aerospace ships via UPS or
FedEx and can ship worldwide. Check your shipper delivery zone time chart to determine the duration of shipping time to and from Montgomery AL.
The magneto is disassembled by a magneto technician so that the parts can be inspected and cleaned.
The magneto parts are cleaned and polished as needed. Magneto frames and housings are not repainted but can be repainted if the magneto is upgraded to an overhaul. The magneto parts are
inspected by visual and non-destructive testing, and must meet the criteria established in the Bendix
and Slick Maintenance and Overhaul manuals.
The 500-hour inspection for Bendix magnetos includes new replacement hardware, new contact points,
oil slinger, oil seal, (2) ball bearings, carbon brush, o-ring, felt strip, felt washer, and impulse coupling spring. The 500-hour inspection for the Slick magnetos includes new replacement hardware, new
contact points, and cam, oil seal, (2) ball bearings, carbon brush, distributor gear to comply with

Champion Slick Service Bulletin SB1-15A and impulse coupling spring. An FAA Form 8130 will be provided to document the 500-hour inspection.
Depending upon the condition and service history of the magneto, extra work or parts may be required.
Some of the extra parts would include worn distributor gear, worn distributor block, faulty coil, faulty
capacitor, worn impulse coupling, worn rotor shaft, or parts affected by airworthiness directives.
If any other parts are required, the customer will be contacted for additional charge approval prior to work
being completed.