An electric starter or backfiring 4-stroke engine will not loosen this Spinner Nut. Simple to install, hold the Spinner Nut with a 22 mm (7/8") wrench and then tighten the socket set screw against the shaft. Required but not included: hex wrench. Engine Shaft: M8 x 1.25. Hex Wrench: 4mm. (5/32 in).