Otto Explains — Car Technology
Timing Belt, Timing Chain or Drive Belt: What’s the Difference?
Short Answer
A timing belt or timing chain keeps the engine’s crankshaft and camshafts turning in step, so the valves open and close at exactly the right moment. The drive belt, also called the serpentine or auxiliary belt, is a separate belt that powers accessories such as the alternator, the AC compressor and, on some engines, the water pump. A timing belt is replaced at the interval your manufacturer sets, whereas a chain is designed to last the life of the engine but can wear and lengthen. If you suspect a timing belt has snapped, don’t keep trying to start the engine.
What does each one do?
Inside the engine, the crankshaft turns as the pistons move, and the camshafts open and close the valves. The timing belt or timing chain links the two so they stay in sync, and each valve opens at the right point in its cylinder’s cycle. Depending on the engine, the same belt or chain can also drive parts such as the oil pump, a balancer shaft or the water pump. A timing belt is a toothed rubber belt, usually outside the engine behind a cover. A timing chain is metal, sits inside the engine and is lubricated by the engine oil. The drive belt has a different job: it runs around pulleys at the front of the engine and powers the accessories, commonly the alternator, the AC compressor and the power steering pump, and on some engines the water pump. An engine has either a timing belt or a timing chain, and most modern engines also have a ribbed drive belt with an automatic tensioner.
Why do some engines use a belt and others a chain?
Both are common, and the choice is part of the manufacturer’s engine design. A chain drive costs slightly more but can carry higher loads and engine speeds, and it is designed to last the life of the engine. A belt has its own advantage: research by a belt-drive supplier found that timing belts run with noticeably less friction than chains, which can help fuel consumption. Some engines use a third option, a wet timing belt that runs in oil inside the engine. On those engines the correct oil grade matters to the belt as well as the engine, because the wrong oil can wear a wet belt out early.
Why does a timing belt need replacing?
A timing belt is a wear part with a replacement interval set by the vehicle or belt manufacturer. Going past that interval risks the belt failing, and on many engines a failure can cause serious engine damage. There is no universal figure: intervals vary from engine to engine, and the published figures assume average use, so a belt can wear sooner depending on driving and temperature conditions, which is worth keeping in mind in a hot climate. A timing belt also tends to fail with little or no warning, so the schedule is a better guide than the belt’s appearance. On many cars the timing belt drives the water pump too, and a drive belt that is breaking up can shed fibres that get caught under the timing belt and make it jump. When the belt falls due, and what is usually replaced alongside it, is covered in what a major service includes.
Does a timing chain last the life of the engine?
It is designed to, and most chains have no scheduled replacement in the way a belt does. A chain can still wear, though. The wear happens in its joints, between each pin and bush, and as they wear the chain grows slightly longer: this is what mechanics call chain stretch. The tensioner keeps the chain taut but cannot make up for the extra length, so the valve timing gradually drifts, which can affect power, fuel consumption and emissions. A heavily worn chain becomes noisier and, in the worst case, can jump a tooth on its sprockets, letting valves and pistons collide, or even break. The chain’s joints are sensitive to the quality and cleanliness of the engine oil, so using the oil your manufacturer specifies and keeping to your oil change schedule helps look after it.
What happens if a timing belt snaps?
The engine cuts out and cannot run again until a new belt is fitted. What happens inside depends on the engine design. In an interference engine, the pistons and the open valves use the same space at different moments. When the belt snaps, the camshaft no longer turns but the crankshaft keeps moving, so a piston can strike a valve that is held open, bending valves and sometimes damaging the pistons or cylinder head. In a non-interference engine there is room for both, so serious damage is less likely, although the car still has to be recovered. Whether your engine is an interference design depends on the specific engine. If it happens while you are driving, pull over safely and arrange recovery. If you suspect the timing belt has snapped, don’t keep trying to start the engine: it cannot run without the belt, and turning it over moves the pistons while the valves cannot get out of the way, which could add to any damage.
Which noises point to a belt or a chain?
A squeal or chirp from the front of the engine often comes from the drive belt slipping. Common reasons are a worn belt, incorrect tension, oil or coolant on the belt, a weak automatic tensioner or a failing pulley bearing. A rattle in the first moments after a cold start can come from a timing chain that has lost tension or stretched: a MINI service bulletin describes exactly this on one engine, and also tells technicians to rule out other sources first, including a brief start-up noise from the oil pump that is normal. A timing belt rarely gives an audible warning, but a rubbing noise from the timing belt cover is a reason to switch the engine off and have it checked. A deeper knock from inside the engine is a different matter, covered in why an engine knocks.
What happens if the drive belt slips or breaks?
Everything the drive belt powers is affected. A slipping belt can show up as weak AC, inconsistent power steering, reduced engine cooling or a warning light, and because the belt turns the alternator, a slipping or broken belt is one of the reasons a battery warning light can come on. If the belt breaks, the accessories it drives stop working: the AC goes off, and you can also lose power steering assistance or, where the belt drives the water pump, coolant circulation, which leads to overheating. If the temperature gauge climbs or a red warning light comes on, stop safely and switch the engine off. Modern drive belts rarely crack visibly; they wear by losing material from their ribs, so a belt can look fine and still slip. The automatic tensioner is a wear part as well, and a weak one can damage the alternator, water pump or AC compressor.
When should a professional take a look?
Have the car checked if a drive belt squeals, if you hear a rattle from the engine at start-up, if the engine misfires or loses power, or if your car is nearing the timing belt interval in its service schedule and you are not sure the work has been done. A technician can confirm whether your engine has a belt or a chain and what its manufacturer’s schedule requires. AutoTronix Express replaces timing belts at the interval your manufacturer sets, and replaces drive belts either as a job on their own or as part of a major service; it also checks them when it tests the alternator and starter. AutoTronix Garage handles timing chain and timing system faults, such as a stretched chain, a worn tensioner or guides, and diagnoses the fault before any work begins.
Common Questions
- How do I know whether my car has a timing belt or a timing chain? Look in the owner’s manual or service schedule: an engine with a timing belt has a belt replacement item in its schedule, while a timing chain normally has none. If the records are unclear, a technician can confirm it from the engine.
- Can I tell when a timing belt is about to fail? Not reliably. Timing belts often fail with little or no warning, and the belt is usually hidden behind a cover. Replacing it on the manufacturer’s schedule is the dependable protection.
- Is a short rattle at start-up always the timing chain? No. A chain that has lost tension or stretched is one possible cause, but other parts can make similar noises, and one manufacturer’s bulletin tells technicians to rule those out before replacing anything. A rattle that keeps coming back is worth having diagnosed.
- Is a squealing drive belt safe to ignore? It is better not to. A squeal usually means the belt is slipping, which can weaken charging, cooling or AC, and a worn belt or tensioner can eventually fail. Have the belt and its tensioner checked together.