What is an interference engine?

So what does it mean if an engine is 'interference'?  Engines are often classified as interference or non-interference. 

Interference relates to whether the valves and pistons can contact each other.

In an interference engine, the possibility exists of the pistons and valves coming into contact with each other.  For example, if the timing belt or timing chain breaks or slips or jumps timing, the piston rising in the cylinder may hit one or more valves that are not completely closed.  (Contrast with the non-interference engine, where this type of contact cannot occur).

Intake valve to exhaust valve inteference

On engines with separate camshafts for the intake valves and exhaust valves, on some designs it is also possible for the intake and exhaust valves to hit each other.  A DOHC (dual overhead cam) engine is often an example where the intake and exhaust valves in a given cylinder could also interefere with each other.

Repairing bent valves

To repair bent valves typically requires removing the cylinder head.  On many vehicles the engine must be removed in order to remove the cylinder heads.  If any of the pistons were dinged, further engine disassembly is required to replace the damaged piston(s).

Cost of repairing bent valves

The cost of these repairs is often USD$1000 on up.  Therefore, on interference engines with a timing belt, it is important to pay attention to the recommended change intervals from the vehicle's Owner's Manual.  Note that timing belt change intervals often include a time limit as well as a mileage limit.

Which vehicles typically have inteference engines

Typically later model vehicles have interference type engines.  Interference designs can have a higher compression ratio and other characteristics desireable to vehicle manufacturers.  Vehicles up through the mid 1990's or so often had non-interference engines.