The spark plug socket is designed specifically to remove and replace spark plugs. It is deep enough to accommodate a spark plug and has a flexible sleeve inside it to grip the spark plug and permit one handed operation. Since various size spark plugs exist, a variety of spark plug sockets are available. For example, the spark plugs used by the Briggs & Stratton 3.5 HP model 91200 engine uses a socket of 13/16" across the flats.