High compression racing engines require ''indexing'' of spark plugs to prevent contact between the piston and the ground wire of the spark plugs. Fits both flat and tapered seat plugs. Once a given plug is selected (placed in a cylinder head and found to have the correct ground wire position), it is placed in the Indexer and the position of the ground electrode marked on the black anodized surface. This mark then provides a reference for checking other plugs to be used in the same cylinder.