Lawn mower ratchet key. The ratchet key engages the drive shaft to the drive gear to turn the wheel. A small spring keeps tention on the key. If the key is not engaging the gear it may be worn and will need to be replaced. It is a good idea to also check the spring and replace it if necessary.
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.
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