What is an interference engine?
Many OHC engines are interference engines, while a few are free-wheeling engines. This refers to the “interference” between valves and pistons. An interference engine will have piston to valve contact when the cam-to-crank timing is out of synchronization, which happens when the timing belt (or chain) jumps or breaks. An interference engine can damage the valves and /or pistons and sometimes cause catastrophic damage to the engine when cam-to-crank timing synchronization is lost.
A free wheeling engine has enough clearance between the valves and pistons that when the cam stops, the pistons will not hit the valves. We recommend changing the timing belt as regular maintenance according to your owner’s manual, or if you are doing repairs such as a head gasket job close to the recommended replacement interval, you should consider changing the timing belt while the engine is disassembled.