Engine's design has pluses
Q If the timing belt breaks in an interference engine, it will self-destruct. What advantage is gained by this design that outweighs the risk of engine damage if or when the belt fails?
— R. Fischer
A This is an important topic because many engines can incur significant internal damage should an aging timing belt break. Automotive and light truck engines use either a chain, belt, or in rare cases, gears to synchronize the camshafts 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 longer-lasting than timing belts. An engine can have one, two or four camshafts.
Many engines are designed such that the valves can collide with each other or the top of the piston, should the belt or chain fail, and the cams and crankshaft lose synchronization. This is known as an interference engine, and is a compromise of performance and failure likelihood and consequences.
Engine performance is all about breathing and a higher-than-typical compression ratio. Superior breathing requires large valves that open deep into the combustion chamber and high compression means a smaller-than-typical combustion chamber. This means the valves may need to extend into the area swept by the piston, and that's where interference may occur. The worst case I've heard was around $6,000 in damage to an engine because of a failed timing belt; in most cases it may be around half that or less.
Timing belts are highly durable, and failures are rare. Most automakers recommend belt renewal at 90,000 to 120,000 miles to play it safe. Belt replacement can be a big job, with a cost of $500 to $1,000, as the water pump, belt tensioners and other parts are replaced at the same time. This is prudent, as anything that's driven by or contributes to belt tension could cause a failure that breaks or derails the belt. Obviously, a service item such as this should be a consideration when purchasing a used car.
In the case of a non-interference or “free-wheeling” engine, the worst thing that should happen, if the belt fails, is the engine will simply stop running. This certainly has other consequences, from a safety and convenience standpoint. In rare cases, a free-wheeler can also incur bent valves or damaged pistons, as carbon buildup can decrease clearances between the conflicting parts.
I don't mean to spread fear, but this topic is an easy one to forget about when a car is seven or eight years old. If one is tempted to stretch the timing belt replacement interval, it would be wise to assess the risk.
Gates Corp. has a convenient look-up tool that indicates if an engine is of interference or free-wheeling design: http:/// bit.ly/PnCfzn.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at firstname.lastname@example.org; he cannot make personal replies.