What are timing belts and timing chains, and how do they differ from one another? Well, the simple answer is that one is a belt, and one is a chain. Of course that’s not an overly helpful answer. You also want to know what exactly it is that they do, so let’s start by talking a bit about engine timing, which is the reason why your car needs a belt or chain.
The basics of mechanical engine timing
Most cars today have four-stroke gas engines. This is because the combustion process has an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. Throughout the four-stroke cycle, the camshaft spins once and the crankshaft rotates twice. The relationship between the rotation of the camshaft and crankshaft is what’s known as “mechanical timing.” It’s what controls the movement of the pistons and valves inside your engine cylinders. The valves have to open at a precise time in conjunction with the pistons, and if they don’t, the engine won’t work properly, if at all.
Around the middle of the 1960s, Pontiac devised a straight-six engine that is the first car built in America that had a rubber timing belt. Previously, virtually every four-stroke engine was equipped with a timing chain. The advantage to the belt is that it is very quiet. They’re also strong, but will wear out. Most automobile manufacturers recommend a timing belt replacement every 60,000-100,000 miles. Now that you know the function of the timing belt, we probably don’t have to tell you that there is never any good outcome if you end up with a broken timing belt.
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.
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 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.
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 belt could break and not cause internal damage, because the pistons and valves are never in the same place.
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.
What happens when a timing belt or chain is damaged?
With proper maintenance, it’s not likely that you’ll experience problems with your timing belt or chain. But when it does happen, as we’ve said, there is no good outcome. So what exactly happens?
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.
Maintenance is vital. If your vehicle manufacturer recommends replacing the belt or chain at regular intervals, do it. Letting it go is very risky, and depending on the age of your vehicle, could result in repairs that will cost far in excess of the car’s actual value. If you’ve bought a used vehicle and you’re not sure if your timing components have ever been checked, have a mechanic check the vehicle over.
This article originally appeared on YourMechanic.com as What Is the Difference Between a Timing Belt and a Timing Chain? and was authored by Valerie Johnston.