If extracting maximum horsepower is your goal, consider adding spark plug indexing washers at your next tune up. Indexing shims allow you to properly orient the spark plug electrode within the combustion chamber of the cylinder head. Proper electrode orientation ensures maximum efficiency (complete burn), increases the speed at which the fuel is burned (more power) and ensures the piston dome does not contact the spark plug electrode (on super high compression engines).


The object of indexing spark plugs is to position the electrode (ground strap) towards the nearest cylinder wall, away from the middle of the chamber. This helps to unshroud the flame kernel that's produced by the ingition arc, allowing it to spread quickly for a faster burn. Due to manufacturing processes, there is no guarantee the threads on any two spark plugs are positioned the same way, or that the threads in the cylinder head are consistent from chamber to chamber. In other words, chances are if you screw a plug into the head, it won't put the electrode in the same position as all the others.


Our shim sets typically come with a variety of washer thicknesses, enabling you to stack the shims to set the proper depth and orientation of the electrode. Each set comes with detailed instructions on how to index spark plugs. This process is typically done with the cylinder head off the engine, however this indexing tool can be used on an assembled engine if you already know the proper electrode depth.