How to properly service spark plugs

How to properly service spark plugs

June 28, 2018 0 By Ray Bohacz

Though the diesel engine is the main worker on the modern farm there are still many gasoline power plants employed for various tasks. By design they have a spark plug in every cylinder.


As simple as a spark plug appears it often is the recipient of improper service that may result in poor performance, short service life, or a misfire under load.


Residing in the combustion chamber of the engine, spark plugs endure severe temperature fluxes, extreme pressure, electrical degradation, and chemical attack from the components blended into the gasoline. Despite the environment the spark plug can deliver a very long life if installed properly. Here are a few tips to help a spark plug perform at its best in any gasoline powered farm engine.


Before removing the old spark plug use a compressed air gun to blow away any dirt or debris that may have collected around the exterior. The dirt cannot be allowed to enter the cylinder bore since it will score the wall and wear the rings.


If the spark plug does not have a tapered seat then a washer-style gasket is required. The gasket is installed with the small side toward the cylinder head. The length of the threaded area is called the reach and positions the electrode properly in the cylinder bore.

Always inspect new plugs for damage. If dropped, check for cracks in the porcelain.  Make sure the side electrode lines-up with the center electrode and is straight. If not, gently reposition it with needle nose pliers.


Use a spark plug feeler gauge to obtain the proper gap. When correct, the gauge will drag evenly on both the center and side electrode.


Make sure the gauge is straight and not cocked when checking. If the side electrode needs to be excessively angled up or down to create the proper gap, the spark plug is the wrong design or the specification is incorrect.


Using carburetor cleaner spray the cavity around the center electrode while having the plug face downward so any material will fall out. This will remove any porcelain dust or debris from manufacturing that can cause a misfire, especially under load.


Coat the thread with anti-seize compound. This is especially important with an aluminum cylinder head or equipment that is not stored in a building. This will aid in removing the plug in the future without galling the threads in the cylinder head. Keep the anti-seize compound away from the electrodes. If it gets on the electrode wipe it off with a clean rag that is moistened with carburetor cleaner.


Coat the secondary wire connection and the porcelain insulator where the boot sits with dielectric compound. It can be found in most auto parts or farm supply stores. This will deter corrosion and prevent the boots from sticking to the insulator and the wires breaking during future service.


Thread each plug in by hand and then tighten with the proper socket and wrench. The plug needs to be tight against the cylinder head to maintain heat transfer and keep the bore sealed. Never use air tools to remove or install spark plugs.