The timing belt runs through a series of pulleys that have belt tensioners in place. As you might gather from the name, the function of a belt tensioner is to keep the proper tension on the belt at all times. They usually wear out at the same time as the belt, and are replaced along with a belt replacement. Most manufacturers and mechanics also recommend that you replace the water pump at this time as well. That’s because the water pump is also usually the same age, and will generally wear out at about the same time.
The timing chain looks similar to 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.
So, how do you know if your car has an interference engine or a non-interference engine? You will likely have to ask your dealer or your mechanic.
A timing belt will usually break when you’re either starting or shutting down your engine. That’s simply because this is when the tension is greatest on the belt. If your engine is non-interference, usually you can get away with just installing a timing belt kit. If it’s an interference engine, there is almost certainly going to be some damage. How severe will depend on the engine speed at the time the belt is thrown. If it happens on shutdown or startup, you’ll probably end up with some bent valves and/or broken valve guides. If it lets go at a high rpm, though, the valves will likely break off, bounce around the cylinders, bend the connecting rods and destroy the piston. Then, as the piston disintegrates, the connecting rods will begin to poke holes in the oil pan and the engine block, ultimately cutting the engine apart. If you think that sounds as though repairs won’t be possible, you’re right.
Now, on to the timing chain. If the chain breaks at a low speed, it might just slide off and not cause any damage. You just install a timing chain kit and you’re good to go. If it breaks or comes off at a high rpm, it will destroy pretty much everything it comes into contact with. Repairs may be possible, but they’ll be costly.