Interference engine

By contrast, in a non-interference engine, the piston does not travel into any area into which the valves open.

Interference engines rely on timing gears, chains, or belts to prevent the piston from striking the valves by ensuring that the valves are closed when the piston is near top dead center.

[1] In interference engine designs, replacing a timing belt in regular intervals (manufacturers recommend intervals ranging from 60,000 to 104,000 miles) or repairing chain issues as soon as they are discovered is essential, as incorrect timing may result in the pistons and valves colliding and causing extensive internal engine damage.

The piston will likely bend the valves, or, if a piece of valve or piston is broken off within the cylinder, the broken piece may cause severe damage within the cylinder, possibly affecting the connecting rods.

If a timing belt or chain breaks in an interference engine, mechanics check for bent valves by performing a leak-down test of each cylinder or by checking the valve gaps.

A pair of poppet valves bent by collision with a piston after timing belt failure. The engine was running at 4500 RPM.
Intake valves bent during a timing belt failure incident