Aircraft spark plugs, such as the Champion spark plug or the Autolite Spark plug come in two different barrel sizes:3/4 (example part number RHM40E). Sometimes called "all-weather"5/8 (example part number REM40E)
When ordering aircraft spark plugs it is important to know which one your aircraft engine uses. Your engine may use either size. If you order the incorrect size the ignition lead nut will be the wrong size. By 3/4 and 5/8 we are referencing the size of the thread. Nominal size for a 3/4 thread plug is 3/4 inch. It is not the size of the wrench! Fasteners are never sized using wrench size.Measure the outer diameter of the thread (major diameter) of your aviation spark plug. You measure approximately 3/4 or 5/8 inch. 3/4 plugs use the H in the part number as in RHM. 5/8 plugs use E in the part number as in REM.The video below shows the difference.
