Generally
This plug is mounted on the front edge and or rear edge of the cylinder head. The function is to plug the back of the duct where the exhaust gas cleaning air pump blows air in. You can also plug the holes that occur if the air pump pipes are removed. The measurement of the plug is 11/16″ = 17.45mm. 2 or 4 plugs are needed for an engine depending on whether the air pump pipes are gone or not.
