Timing belts vs timing chains

The camshafts have to turn exactly once for every two turns of the crankshaft, in step with it, for the whole life of the engine. If they slip even a little, every valve opens at the wrong moment, as you saw in Camshafts and valve timing.

Most engines do this job in one of two ways:

• A timing belt: a toothed rubber belt, strengthened with cords of glass fibre or similar, running dry behind a plastic cover on the end of the engine. In the UK it’s usually called a cambelt.
• A timing chain: a metal chain, like a heavier bicycle chain, running in oil inside a metal cover.

Both do the same job, and neither is simply better. A belt is quiet, light and cheap to make, but it has to be replaced on a schedule, and it gives no warning before it snaps. A chain is meant to last the life of the engine, but only if the oil is looked after, and it’s a much bigger job when it does wear out. Knowing which one your engine has is one of the most useful things to find out about any car you own or are thinking of buying.

The demo is the front of a twin-cam engine with the timing cover off. The crank sprocket at the bottom drives the belt or chain round the two Mustard cam sprockets. The cam sprockets have twice as many teeth as the crank’s, so they turn once for every two turns of the crank. Each sprocket has a Paper timing mark, and every two turns of the crank they all line up with the fixed marks on the engine. Press Line up the marks to turn to that point.

Start with the Timing belt and drag the distance up. The belt’s cords barely stretch, so the marks still line up perfectly at 80,000 miles (128,747.52 km). That’s the trouble: a worn belt looks and sounds just like a good one. The change is due by distance or by age, whichever comes first, because the rubber hardens and cracks with heat and time even if the car sits still.

Now press Timing chain. A chain can’t snap from old age the way a belt can, but every link pivots on a pin, and those pins wear. Each one wears only a tiny amount, but a chain has more than a hundred links, so the whole chain gets longer. Engineers call this stretch.

• The tensioner presses on the slack side of the chain and pushes out to take up the extra length. Most chain tensioners are pushed by oil pressure from the engine.
• The stretch on the pulling side lets the cams fall behind the crank. Watch the cam marks drop behind the fixed marks as you drag the distance up.

With the oil changed on time, the chain in the demo has stretched less than 0.3% at 150,000 miles (241,401.6 km), which is fine. Now press Skipped oil changes. Old, dirty oil full of soot and grit wears the pins much faster. By 100,000 miles (160,934.4 km) the cams are 10° late, which is enough for the engine to feel flat and for the ECU to notice that its cam and crank sensors disagree. A little further, and the tensioner runs out of travel: the slack chain rattles, and it could jump a tooth on its sprocket.

The model is simplified, but it shows the real pattern: a belt fails suddenly and on a timetable, and a chain wears gradually, depending on how well the oil has been looked after.

What happens when a belt snaps or a chain jumps depends on the engine.

In an interference engine, an open valve reaches down into the space the piston uses at the top of its stroke. They never meet, because the timing keeps them apart. If the belt snaps, the cams stop with some valves held open, the crank carries on turning, and the pistons hit them. The valves bend, and pistons and guides can be damaged too. On an older car, the repair can cost more than the car is worth.

In a non-interference engine, there’s room for a fully open valve and a piston at the top at the same time. If the belt snaps, the engine stops and won’t restart, but a new belt usually gets it going again.

Most modern engines are interference engines. More lift and higher compression make more power, and both bring the valves and pistons closer together.

Changing a timing belt means taking off the timing covers, and often an engine mount and the drive belt for the accessories. Because of that labour, garages replace the tensioner and the idler pulleys at the same time, and on many engines the water pump, as the belt drives it. A kit usually includes all of these. Intervals vary a lot between engines, from about 40,000 miles (64,373.76 km) to 150,000 miles (241,401.6 km), and from 4 to 10 years, so always check the schedule for your exact engine.

A timing chain has no change interval. It’s checked when it shows signs of wear: a rattle on start-up, fault codes for the cam timing, or a stretch reading taken with a diagnostic tool. Replacing it means new guides, tensioner and sprockets too. On some engines the chain is at the back, next to the gearbox, and the engine has to come out to reach it.

Wet belts run inside the engine, in the oil, like a chain. They’re quieter and have less friction than a chain, which helps economy. But if the oil is the wrong type or left too long, the belt can swell and shed bits of rubber, which block the strainer the oil pump draws through. Ford’s 1.0 EcoBoost and the 1.2 PureTech used in many Peugeots, Citroëns and Vauxhalls are known examples. On these engines, using exactly the oil the maker specifies and changing it on time matters even more than usual.

Gears do the job on some lorry and tractor diesels and a few race engines. They last almost forever and keep perfect time, but they’re heavy, costly and noisy.

How to tell which your car has: the service schedule in the handbook lists a timing belt change if it has one. If you’re buying a used car with a belt that’s near or past its interval, ask for the receipt for the last change. If there isn’t one, budget for a change straight away.

• Snapped belt. The engine cuts out suddenly while running and won’t restart; it may crank faster than usual because there’s no compression. On an interference engine, don’t keep trying to start it, as the valves are probably bent already. It’s almost always an old belt, or one soaked in oil or coolant.
• Oil or coolant on the belt. A leaking cam or crank seal, or a weeping water pump, soaks the belt, and rubber breaks down quickly once it’s wet with them. Fix the leak and change the belt together.
• Failed tensioner or idler pulley. A worn bearing whines or rumbles and can seize, which wrecks the belt. That’s why they’re changed with it.
• Stretched chain. A rattle from the end of the engine for a second or two after a cold start, before the oil pressure pumps up the tensioner. Later, the engine warning light comes on with cam timing or “cam/crank correlation” fault codes. Left too long, the chain can jump a tooth or break its plastic guides, sending pieces into the sump.
• Timing set one tooth out. After a belt or chain change, a sprocket fitted one tooth out makes the engine run badly and set fault codes. Turning the engine over twice by hand and checking the marks before starting it catches this.

The timing drive isn’t where power comes from, but more power and more revs put it under more strain.

• Raising the rev limit with a remap puts more load on the belt or chain and the tensioner. On an engine known for chain trouble, fit a new chain kit before asking more of it.
• A performance cam needs stiffer valve springs, and pushing them open takes more force through the belt or chain. When the cams are out for a cam change, it’s the best time to renew the timing parts too.
• Upgraded tensioners and guides. For engines with a known weak point, aftermarket and updated factory parts are often stronger than the originals. Check for an updated part before buying the original again.
• Adjustable cam pulleys let you fine-tune the cam timing, as Camshafts and valve timing explains.

Whatever you change, a modified engine makes a stretched chain or a tired belt even more costly to ignore: higher revs make valve and piston contact more likely and more violent.