An interference engine is a type of automotive piston engine in which the pistons will make contact with the valve train in the event of a timing belt failure. The other type of automotive engine is a non-interference engine. With this engine, the pistons will not make contact with the valve train if the timing belt would happen to break or slip. It is very important to follow all manufacturer guidelines concerning timing belt or chain replacement with an interference engine. Typically, when a timing belt or chain fails in an interference engine, the engine must be replaced or rebuilt due to the resulting catastrophic damage between the pistons and the valves.
In the common internal combustion engine, the pistons come very close to the open valves on the exhaust stroke. The valve timing is extremely critical in this type of engine in order to avoid valve-to-piston contact. With the non-interference engine, this is typically not an issue with stock pistons. High-performance domed pistons, on the other hand, can come into contact with the valves with improper timing measures due to the increased height of the piston within the combustion chamber. The combustion chamber is so shallow on an interference engine that even stock low-compression pistons will contact the open valves if the timing should go awry.
When a piston makes contact with an open valve, serious damage results. It is common for the piston to be broken from the force of the contact with the open valve. The valves themselves are quite often bent and even broken off of the valve stems, creating havoc as the broken valve bounces throughout the combustion chamber. The steel valve can become a battering ram within the combustion chamber, beating and battering the cylinder head and often breaking through to the internal water jacket. In this scenario, the interference engine may be reduced to a non-repairable collection of damaged parts.
Most automobile manufacturers recommend specific replacement intervals of the timing components in an interference engine. Following the guidelines might prevent the unnecessary replacement of an engine or a major engine rebuild at the very least. There is no specific maintenance schedule with timing components often due to their location inside of the engine; however, proper oil change intervals and valve adjustment will get the most longevity out of all engine components. When an engine has valve train issues and it is unknown whether the engine is an interference engine, it is best to avoid cranking it over.