With proper engine maintenance, intake and exhaust valves usually provide thousands of miles of good service. Engine valve problems begin to occur when your engine is subjected to overheating, lack of lubrication or over-revving. Bent valves will not only damage pistons, but also valve guides, camshafts and valve train components.
If the timing belt on your engine breaks while driving, serious internal engine damage may occur. This is particularly true if you car is equipped with an interference engine. Interference engines have close tolerances between the valves and the tops of the pistons. After a timing belt breaks, the engine continues turning long enough for the pistons and valves to come in contact. This happens because the pistons and valves are no longer in time with each other. The damage may include bent valves, broken pistons and damaged engine heads.
Depending on the engine your car is equipped with, there is a maximum number of revolutions per minute you can safely operate your engine at. When the maximum safe rpm rating is exceeded even for a short time, extensive engine damage can occur, including bent valves. When an engine is over-revved, the valves can "stretch" and come in contact with the pistons. During over-revving, the engine may not maintain proper timing and allow the valves to contact the tops of the pistons causing severe piston damage and bent valves.
Problems such as lack of lubrication and engine overheating can also cause bent valves. If you continue to operate your engine while it is overheated, internal engine tolerances will be reduced to the point that the valves can stick in the valve guides, causing the valves to contact the pistons. Similarly, insufficient lubrication can cause valves to stick in the guides, resulting in the valve being bent when it strikes the pistons. On overhead valve engines, lack of lubrication and overheating can cause lifters to stick, resulting in both bent valves and bent pushrods.
During engine rebuilding, care must be taken to ensure proper clearance is maintained between the pistons and valves and to ensure valve reliefs on piston tops are aligned properly with the intake and exhaust valves. Use caution when milling heads so that the proper valve to piston specification is maintained. Check the valve lift specification before final valve train assembly is complete. If any of these specification are incorrect, you could end up with bent valves when you start up your rebuilt engine for the first time.
