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: