A sheared-off-valve - The weaker component gives way
Piston and valves are the best of buddies—as long as they keep their distance. But they can come to blows if they get too close! Although it really shouldn’t happen, it’s not uncommon for the piston to make contact with a valve that is not fully closed, damaging it or even shearing it off. The consequences can be costly!
An unfair fight: piston vs. Valve
Four-stroke engines can’t manage without them: valves are necessary components for the charge exchange process—that is to say, the exchange of fresh intake air and exhaust gases from the cylinder. If a valve is to remain intact, it is vital that it keeps out of the piston’s way, because a close encounter between these two rarely turns out well!
When the piston moves into the top dead center, the exhaust valves should be closed again by the camshaft and the valve springs. If, however, the piston strikes a valve that is not yet fully closed, the weaker of the two gives way—in this case: the valve.
But why does piston collision occur?
The valve impacts described above are most commonly due to the following:
- Incorrect valve timing: for example, a slipped timing belt or timing chain
- A broken timing belt: perhaps because the replacement interval was too long
- A stretched timing chain: owing to wear to the chain links, for instance
- Overspeed: caused, for example, by accidentally downshifting to an excessively low gear
- Installation error: such as placing the cylinder head on the open valves
- Engine repair error: for example, cylinder head planed and valve depth not adjusted
Consequences of piston collision
The damage varies depending on engine speed, valve arrangement, and the shape of the piston or combustion chamber. In the best-case scenario, only the valves are bent. But it’ll cost you a small fortune if the valve disk breaks off and falls into the cylinder. In that case, the piston and the cylinder head will be badly damaged and both will definitely need to be replaced. Sometimes, the impact of the valve on the piston is transferred to such an extent that even the camshaft and hydraulic followers are affected. If this happens, the only option is a complete overhaul of the engine.
Better safe than sorry
Even if only the valves are bent and no other damage is evident, you should always check all the surrounding components. Deformation and damage to the valve guides, valve seats, and hydraulic followers is not always apparent at first glance, so they must be examined very closely!
As a general rule: even if, for example, only one exhaust valve shows signs of impact, all exhaust valves should still be replaced! Attempt to make savings here and you’ll pay the price later.