Engine Timing and Drive: How Your Engine Keeps Time and Why It Matters

Every reciprocating engine needs the crankshaft and camshaft(s) kept in precise mechanical sync. A few degrees of timing error makes the engine misfire; complete loss of timing means valves meet pistons and the engine ends its life in seconds. The mechanism that maintains this sync is the timing system — belt or chain, with tensioners, guides and idlers. Around it sits the auxiliary drive — a separate belt that runs the alternator, water pump (sometimes), air-con compressor and other accessories.

Two systems, two reliability profiles

Timing belt drives use a toothed rubber belt running over toothed sprockets on the crank and cams. The belt is a defined-interval service item — typically 60,000–100,000 miles or 5–7 years. When it’s replaced, the tensioner, idlers and (on most engines) the water pump are replaced with it as a kit. Belt failure is catastrophic on interference engines (which is most modern engines), so the interval matters.

Timing chain drives use a steel chain instead of a belt. Chains were sold as "lifetime" items but several engine families now have documented chain-stretch and tensioner issues — particularly VAG 1.4/1.8/2.0 TSI early EA888, PSA 1.6 HDi, BMW N47 diesel, Mini 1.6 (PSA/BMW). Treat chain replacement as service work at 80,000–120,000 miles on these.

Auxiliary drive belts

The auxiliary belt (sometimes called the serpentine belt or accessory belt) runs at the front of the engine, driving the alternator, water pump (on some engines), air-con compressor, and power steering pump (on older designs). It has its own tensioner and idler pulleys. Service interval is usually 60,000–100,000 miles. Failure is annoying — alternator stops charging, air-con stops working — but not catastrophic.

What this category covers

Timing belts, timing belt kits, timing chains, timing chain guide rails, drive belts, drive belt tensioners and idlers. The whole assembly that turns crank into cam motion.

Browse the timing and drive range for your engine on the Timing & Drive collection. Service these systems on time and the engine lasts; skip them and the engine doesn’t.