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.

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