2001 Edition Yamaha F225A and LF225A Outboards Service Manual, Page 13Get this manual
Features and benefits Piston The piston has been carved out of forged stockTo prevent the valves from coming in contact with the piston, the piston is provided with valve recessIf the engine goes out of timing and 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