From Mechanical Database
An Interference engine is a type of internal combustion engine where in the event of a serious valve train failure, or a 180 degree timing error, could lead the valves and pistons to strike each other. The most common cause of this a broken timing belt, but a catastrophic timing chain failure can also bring about this situation.
For an engine to work properly, the actions of the pistons and valves must be precisely harmonized. This is accomplished in three ways. In racing engines, gears may connect the camshaft and crankshaft. In street engines, that task is accomplished by a timing chain or belt. When the chain or belt is installed, the camshaft and crank must be turned so that the number one cylinder is at Top Dead Center, or when it is at the maximum of its stroke. Generally, there are timing marks on the timing gears. or pulleys to aid in this, and the gears themselves are usually keyed to prevent misalignment.
If the belt or chain breaks, then this relationship is destroyed, and the valves and pistons move independently. In most automobile engines, this does no serious damage. The motor simply stops running until the belt/chain is replaced and the engine retimed. Even when the valves and pistons are at full extension, a small gap remains between them. This is called a non-interference engine. If the chain or belt breaks in an interference engine, the piston and valve will strike each other. The least this will do is bend the valve. The piston may also be damaged, and bits of metal may work loose in the engine. An engine rebuild is required, with engine replacement a possibility.
How to Avoid a Valve Piston Moment
Timing Chain failures are very rare. A timing chain is basically a short bicycle chain, often with doubled links. They are usually good for well over 100,000 miles. When they fail, it usually involves stretching. Timing chains generally provide a lot of warning before they fail, giving on average a year of driving.
Timing belts are different. They are really nothing more than a wide version of the serpentine belt that operates your car's accessory drives, such as the power steering pump and alternator. They are cogged, to permit precise timing. Timing belts break with little or no warning. While belt life varies, most require replacement every 60,000 miles. 30,000 miles is recommended for Porsche owners. A vehicle's service will contain information on whether the engine has a chain or a belt, and if a belt, its expected life. The manual will also tell you whether or not the engine is an interference design.
If the timing belt or chain is changed on a vehicle the tensioner must also be changed. If a timing chain is being changed, then the gears should be changed as well.
Interference engines with timing belts for a list of all effected vehicle makes.