Briggs & Stratton flywheel key. The flywheel key aligns the flywheel in the correct position on the crankshaft. If the engine stops suddenly due to hitting a hard object, the flywheel key shears in half to prevent damage to the engine. If the flywheel key is broken, the engine won't start.
Screw, 1/4-20 x .5
Cylinder head screw (5/16 - 18 x 2.69)