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:

Do you need assistance? Know before you call.

Do you need assistance? Know before you call.

Kelly Aero receives calls daily from customers in need of product support to select a Kelly product or
technical assistance after the product has been installed. Due to our long industry experience at Kelly
Aero, the vast majority of calls are routine, and we can quickly provide an answer to any question asked
to us. However, calls get a bit complicated when the first several minutes are spent trying to determine
what the topic is when a customer calls.
All too often, customers start their phone call with a vague request of “I have a Lycoming engine and I
need a harness” or “…the magneto on my engine isn’t working…” While all conversations have to start
somewhere, the reality is that some conversations stop as quickly as they start. The information
provided by the customer is missing too many important details or the customer simply does not know
the details. It can be frustrating to a customer who is in a rush or in the middle of working to solve a
problem when the conversation with Kelly becomes a game of 20 Questions. Unfortunately, this game
of 20 Question is particularly frustrating for both sides on a busy Friday afternoon when an airplane
needs to fly on Saturday morning.
Just like how a pilot completes a checklist before the airplane is ready to fly, customers and mechanics
can complete a simple checklist of items prior to calling that will the customer to ask the right questions
quickly, and the Kelly Aero representative to provide an accurate answer.

What is the engine make and model you are working on?
Invariably, the Kelly Aero Product Support person will need the following specific information:

  1. Which company manufactured the engine? Is it Continental, Lycoming, Superior, Franklin,

    As a note to this comment: The name of company who overhauled the engine is not useless,
    but also not particularly useful. Starting the conversation with “I have an engine overhauled by
    Empire Engines” is good to know, but does not really provide any magneto information. What is
    useful is the specific original manufacturer of the engine, such as Continental or Lycoming.
  2. What is the complete engine designation?
    Ok, you know that you have a “four cylinder Lycoming” or a “550 Continental” but specific
    details are critical. For example, the magnetos used on a TSIO-550-K are much different in
    configuration and troubleshooting than the magnetos installed on an IO-550-A. Provide as
    much exact information as possible including the prefix, engine number and suffix, such as O-
    320-D2J, O-470-R, TSIO-550-C.
    Additionally, some of the Kelly Aero Technical reps are experienced A&P mechanics with deep
    knowledge of specific engine and airframe applications. If a customer calls in with what seems
    to be a magneto operation issue, a bit of discussion may lead to other known areas of concern
    unique to specific engine models.
    What is make and model of magneto?
    Simply referring to a magneto “a Kelly mag” or maybe as a “Bendix” or “Slick” usually is too broad of a
    description. No surprise, but Kelly will need the exact magneto part number details to put the known
    service history or troubleshooting techniques into perspective.
  3. Is the OEM manufacturer of the magneto Bendix or Slick?
    While Kelly takes it as a compliment when a customer says, “it’s a Kelly Aero mag”. While Kelly is
    an OEM for replacement parts, we are also a service provider to overhaul OEM magnetos. The
    magneto data plate may be labeled with Kelly Aero, but the magneto it is attached to is Bendix
    or Slick magneto, but overhauled by Kelly Aero.
  4. What it is the magneto model series? Bendix makes the 20 series, 1200 series, and Dual D2000
    and D3000 series magneto. Slick mags will be either a 4300 or a 6300 series. Drilling deeper,
    the part number of the magneto is critical as this identifies the configuration of the magneto. As
    an example, the Bendix 20 Series 10-51360-37 and the 10-500514-1 are approved for the same
    engine applications, but very different in the details of how the magnetos are mounted to the
    engine and how the ignition switch p-lead attaches. But, these seemingly small details can add a
    lot of time and expense to an otherwise simple installation if a mechanic has to alter a P-lead or
    order different mounting clamps and gaskets.
    What is the magneto serial number?
    The new data plate that Kelly Aero affixes to all Bendix and Slick overhauled magnetos has lots of
    information that the Kelly Aero product support representative will need to know. Most important will
    be the serial numbers stamped on the data plate.
    The Kelly Aero data plate will display both the OEM issued serial number and the Kelly Aero issued serial
    number. The OEM serial number is important to track any OEM specific service issues or compliance to
    OEM Service Bulletins. The Kelly Aero assigned serial number links the overhaul of the magneto to
    records maintained by the Kelly Aero Repair Station quality management records. The Kelly Aero serial
    number is required to confirm compliance to Airworthiness Directives, certain manufacturer Service
    Bulletins and the general timeframe when the magneto was serviced.
    When was the magneto installed? Calendar date and hours, please…
    Hours of operation and calendar time are two different ways to measure time in service for magnetos,
    and each tells a story independently, and together to put the magneto service history into perspective.
    For example, calendar time frames the reference for the type of parts or service issues known to be in
    effect during a particular year or month. Hours in service provides a view of how quickly, or slowly,
    hours were accrued on the magneto.
    The relationship of calendar time and hours is very important for the Kelly Aero product support
    specialist to assist with customer questions. For example, a customer may comment that they have a
    question regarding a magneto with 100 hours. However, was that 100 hours accrued over two months
    or ten years? Low magneto time is one story, but low magneto time over many years tells a different
    Harness Nuts and Spark Plug Type. Let’s have this discussion again…and again…and again…
    Search through Kelly Aero ignition system articles and ignition harness selection has been a topic more
    than once. Ignition harnesses have a few options and do required the buyer to do some homework as
    part of the selection process. Virtually all customers are able to determine which custom fit harness is
    required for their engine using Kelly’s Ignition Harness Application Data by clicking on this link:

But, there are also many, many times that customers, typically private owners who are learning the
process to buy their own parts, who need a bit of guidance on selecting between ignition harnesses that
fit Bendix or Slick magnetos. As always, the most confusing part is the difference between 5/8-24 or
3/4-20 spark plug nut connections.
Sorting out the difference between spark plug nut sizes is usually the substance of most phone calls.
The 5/8” and 3/4” refers to the diameter and thread pitch of the spark plug barrel, not the wrench used
to remove the nuts. The easy way to determine nut size follows:

  1. A 3/4” wrench is used to remove 5/8”-24 spark plug nuts used on REM style spark plugs
  2. A 7/8” wrench is used to remove 3/4”-20 spark plug nuts used on RHM style spark plugs
    Who is asking the question: an A&P mechanic….or an Owner?
    Let’s be realistic: Aircraft owners are more deeply involved in the maintenance process of their
    airplanes than ever before. Owners provide important operational information used by mechanics and
    product support representatives. Owners research and purchase parts used on their airplanes,
    consulting their mechanic, parts supplier, and manufacturer sales and support teams. Internet owners
    groups can be useful, but very often lead to some good answers. The same groups also generate
    answers that may not be completely accurate or even completely incorrect. Ultimately, owners are
    going to have to spend their hard earned money and the educated owner is motivated to do research on
    the products that they own or want to buy.
    By no means does this imply that Kelly Aero does not want to talk to aircraft owners. In fact, we
    encourage and support aircraft owners to contact us to work through product selection or product
    operation questions. But, calls can become frustrating when the owner is speaking in terms which are
    not aligned with standard industry terminology used by trained mechanics. Additionally, Kelly Aero
    product support answers may overwhelm an untrained caller with all sorts of terms and references that
    will make no sense. It usually require a lot of repetitive discussion to explain terms and concepts that
    are easily understood, with no further explanation, by an A&P mechanic.
    From the technical support perspective, Kelly Aero can be less technical, use or explain more of the
    specific terms that an owner may not understand. In effect, Kelly Aero can tailor our conversation with
    the customer to speak with terminology useful to the caller. The end result is a much better experience
    for both the customer and the Kelly Product Support representative.
    Kelly Product Support does not replace required reading material!
    When working on a critical process, such as ordering internal components to the magneto, completing
    magneto assembly, or performing magneto to engine timing, the Kelly Product Support Representative
    will ask, “Do you have a manual?”
    To be blunt, working on a magneto is not the same set of skills as required to change the light bulb
    inside your refrigerator. The magneto and ignition harness are components critical to the safe operation
    of the engine. It only makes sense that anyone who plans to or is working on an item critical to safe
    operation would have a set of instructions in their hands. It is reasonable and obvious that a properly
    rated individual should have access to and use the service manual. Nevertheless, it is also a good idea

for an educated owner to read the service manual to have an idea of the work required for a particular
While Kelly Aero is eager to assist our customers, discussions with a Kelly Product Support
Representatives do not replace the need- or FAA requirement- to have a service manual for the
component that will be serviced. Copyright laws restrict Kelly Aero from sending copies of OEM service
manuals to customers, but Kelly can distribute our own documents freely.
A brief note regarding Kelly Aero manuals: Keep checking the Kelly Aero web page during 2021! We are
working on new product application manuals, product service manuals, and supplementary Service
Letters to make it easy for users of Kelly Aero products to service all of the parts and assemblies that we
Are you ready to make the call?
Get your notes ready and give Kelly Aero a call at 334-286-8551!