How It Works: Timing Chains and Timing Belts

Article content

Inside the front of your engine, there’s a component that’s either a timing belt, or a timing chain. They both serve the same function, but they’re made of different materials. And while belts aren’t as common as they used to be, if your engine has one, you need to factor in some preventive maintenance.

Article content

A timing chain will usually last the life of your engine, but a timing belt should be replaced at the appropriate interval. If a new one isn’t installed when recommended, the old one could wear out and break, and leave you stranded.

We apologize, but this video has failed to load.

Try refreshing your browser, or
tap here to see other videos from our team.

How It Works: Timing Chains and Timing Belts Back to video

Engine timing refers to the pistons, valves, and spark plugs all doing their thing at precisely the right moment. In a four-stroke engine – the type used in all modern vehicles – each piston moves four times to complete one cycle.

It first moves down in the cylinder to create a vacuum. As it does, the intake valve (or valves, depending on the engine design) opens to let in a mixture of fuel and air. In a direct-injection engine, only air comes in through the valve, while the fuel is squirted directly into the cylinder.

Article content

The valve closes, and the piston moves up to compress the air/fuel mixture. The spark plug fires and ignites the fuel, and that energy pushes the piston back down. It’s attached to the crankshaft and this action turns it; the crankshaft’s spinning motion is then transferred to turn the vehicle’s wheels.

Finally, the exhaust valve opens, the piston moves up to push out the exhaust gases, and then the cycle starts all over again. This entire operation happens hundreds of times per minute.

The valves must open and close at precisely the right time. If they don’t, the engine won’t run properly, and that’s where the timing chain/belt comes in.

The intake and exhaust valves are activated by the camshaft, which has lobes that push them down to open and then let off so they spring back to close. Depending on the engine, there will be one, two, or four camshafts. Each one has to turn in harmony with the crankshaft’s revolutions, rotating at half the crankshaft’s speed.

Article content

There are sprockets at the end of the crankshaft and camshaft, and the timing chain or belt is attached to them. As the crankshaft turns, the belt/chain turns the camshaft at precisely the right speed. On most engines, it also operates the water pump, which circulates coolant through the engine and radiator.

A timing chain is exactly as it sounds – it looks a lot like a bicycle chain – while timing belts are usually made of reinforced rubber compound. Chains, which used to be the only choice, are stronger and more durable. About the only issue you’re likely to ever see is an oil leak from the cover that goes over it, which can happen when the gasket gets old.

Belts became popular because they’re quieter, cheaper, and lighter, a consideration when automakers shave whatever weight they can for fuel efficiency.

Article content

Thanks to the availability of more lightweight materials, most auto manufacturers are switching back to chains, which don’t require maintenance. But many older vehicles still have them, and may now be near their recommended replacement, which is usually around 150,000 to 175,000 kilometres.

You can’t tell which you have by looking, as it’s hidden. The rubber belt you see under the hood is the serpentine belt, which runs such components as the alternator and air conditioning compressor. Check the owner’s manual for the scheduled maintenance to see if a timing belt is listed (it’s separate from “drive belts,” which are the visible ones).

Timing belts are replaced proactively because they can’t be easily examined for wear. If the belt breaks, there’s no connection between the crankshaft and camshaft. The valves won’t open and close, and the engine won’t run.

Article content

On most vehicles it just means you’re walking, but a few older and collectible models, including some performance engines from the 1990s and early 2000s, had an “interference” design. When the valves open, they move so far down they’re within the piston’s travel area, which allows in more air for more power. Of course, by the time the piston gets to the top, the valve’s already vacated the space. But if the timing belt breaks, piston and valve can meet, resulting in expensive damage that, in the worst-case scenario, may require a new engine.

The big issue with replacing a timing belt is that it isn’t cheap, and most of the money is in the labour. Not only does the front engine cover have to come off, but the technician has to get to it first. Any other components in the way will also have to be removed, and on some, the engine itself might have to be moved.

The shop might suggest replacing the water pump at the same time, but the intention isn’t to pad the bill with an extra part. The water pump is a wear item – and since it takes the same amount of work to get to it, replacing it now means you won’t be paying that big labour bill twice if it needs replacing in future.