Way back in the mid-1960s, Pontiac came out with an ove […]
Way back in the mid-1960s, Pontiac came out with an overhead cam, straight-six engine that claims to be the first American car with a rubber timing belt. This automobile engine was really quite an impressive design at the time, and it is too bad that so many of them have been pulled out and discarded in exchange for "boring" V8s. Before Pontiac unveiled this engine design in 1966, nearly every four-stroke engine used a timing chain. Automotive timing belts then and now are made of rubber, often with fiberglass or Kevlar woven into them for extra strength. They are known to be very quiet as they rotate, but they do wear out over time. Most vehicle manufacturers recommend that timing belts are changed between 60,000 and 100,000 miles, though there are exceptions, both higher and lower.
Timing belts frequently snake through a series of pulleys and have mechanical or self-contained hydraulic timing tensioners to keep the proper tension on the belt throughout the entire rpm range. These mechanical and hydraulic tensioners and pulleys should always be replaced at the same time as the belt. They are all the same age with the same mileage, so replacing them is a great habit and great insurance. Along with pulleys and belt tensioners, water pumps are often replaced at the same time as timing belts. This is for two reasons. Some water pumps are actually driven by the timing belt itself, and are therefore a liability if they are old and worn out. The other reason is because many water pumps, especially on newer vehicles, are tucked behind the timing belt due to packaging constraints. You wouldn't want to replace the timing belt and then have to take it off again later when your old water pump fails. Doing the job twice is never fun.
Timing chains serve the same purpose as a belt, but usually lasts quite a bit longer. Some manufacturers suggest replacing it at certain intervals, but others state that it will last as long as the car itself.
The timing chain looks exactly like a bicycle chain, and as you might expect, it’s noisier than a belt. The other problem with timing chains is that if they do break, they’ll usually cause a lot more damage than a broken belt. Not that we’re suggesting a broken timing belt isn’t going to cause you problems – it definitely will. But with a broken belt, you might get away with just having the heads done. A broken chain will most likely result in damage that will result in a complete engine transplant being less expensive than the repairs you’ll need.
A timing chain also has tensioners that keep it in place, but unlike belt tensioners, the timing chain tensioners are controlled by the oil pressure in the engine. So, if your oil pressure becomes too low for some reason, the tensioners will fail, the timing will offset, and the chain will most likely fail in a spectacular fashion. The advantage, though, to chains is that they have nothing to do with your water pump, so you don’t usually have to replace the pump at the same time you replace the chain.
No discussion of timing belts and timing chains would be complete without a few words about interference engines. An interference engine is one in which the valves and pistons occupy the same space in the cylinder – but not at the same time. It’s a very efficient type of engine, but if you’re slack about maintenance, you could run into trouble. If you end up with a broken timing belt, the valves and pistons could end up in the cylinder at the same time. We probably don’t need to tell you that this would be a very bad thing. On a non-interference engine, the Rubber Timing Belt could break and not cause internal damage, because the pistons and valves are never in the same place.