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Under the Hood: Interference vs. noninterference engine

| Saturday, July 19, 2014, 12:01 a.m.

Question: I want to find out how I can identify whether a car's engine is interference or noninterference from a layman's point of view; that is, some signs or writings on the vehicle.

Answer: This is an important topic, because many engines can incur significant internal damage should a timing belt break. Automotive and light truck engines use either a chain, belt or, in rare cases, gears to synchronize the camshaft and crankshaft. A camshaft operates an engine's valves, and the crankshaft transfers the up and down motion of the pistons and rods to the transmission, and ultimately to the wheels. Belt-drive systems have been popular on overhead-cam engines because they're inexpensive, quiet and lightweight. Chain-drive systems are making a comeback on many newer engines because they're sturdier and last longer than timing belts. An engine can have one, two or four camshafts.

Many engines are designed so the valves can collide with the top of the piston should the belt or chain fail, and the cam and crankshaft lose synchronization. This is known as an interference engine, and is a compromise of performance vs. belt failure likelihood and consequences. Engine performance is all about breathing and a high compression ratio. Superior breathing requires large and/or multiple valves that open deep into the combustion chamber, and high compression means a smaller-than-typical combustion chamber. This means the valves need to extend into the area swept by the piston, and that's where interference may occur. A failed timing belt can cause damage costing several thousand dollars to repair.

Timing belts are highly durable, and failures are rare. Most automakers recommend belt renewal somewhere between 60,000 and 120,000 miles to play it safe. Belt replacement is not cheap, ranging from $500 to $1,000, as the water pump, belt tensioner and nearby parts are often replaced at the same time. This is prudent, as anything that's driven by or contributes to belt tension could cause a failure that could break or derail the belt. Timing belt replacement should be a consideration when purchasing a used car. If no documentation exists that the belt has been replaced on schedule, it's best to assume it hasn't been and plan accordingly.

In the case of a noninterference or “free-wheeling” engine, the worst thing that should happen, if the belt fails, is that the engine will stop running. This has consequences from a safety and convenience standpoint. In rare cases, a free-wheeler can incur bent valves or damaged pistons, as carbon buildup can decrease clearances between the conflicting parts.

There is no way I'm aware of to determine whether an engine is interference or noninterference via information attached to the vehicle. The majority of modern engines are interference type. Here's a link to the somewhat dated Gates Rubber Co. timing belt application chart, which indicates whether an engine is of interference (star symbol) or free-wheeling design (no symbol): If a vehicle is not listed, it likely has a chain-drive system.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at

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