Yamaha F225A Service Manual page 13

Piston
The piston has been carved out of a forged stock. To prevent the valves from coming in contact with
the piston, the piston is provided with a valve recess. If the engine goes out of timing and a valve
opens when the piston is at top-dead-center, this recess prevents the valve from coming in contact
with the piston, thus preventing engine damage.
1 Valve spring
2 Valve seal
3 Piston
4 Valve stem
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Features and benefits
5 Intake valve
6 Intake camshaft
7 Valve lifter
8 Valve recess
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