a timing chain system is becoming worn include a rattling noise from the front of the engine. Most pushrod engines, where the crankshaft and camshaft are very close together, use a short chain drive rather than a direct gear drive. This is because gear drives suffer from frequent torque reversal as the cam profiles "kick back" against the drive from the crank, leading to excessive noise and wear. Timing gears Timing gears are used in various overhead valve engines, due to the close proximity of the camshaft to the crankshaft. Fiber or nylon covered gears, with more resilience, are often used instead of steel gears where direct drive is used. However, commercial engines and aircraft engines use steel gears only, as these other materials can fail suddenly and without warning. Effects of failure Failure of the timing system will prevent an engine from running. Many modern cars use interference engines, which can suffer catastrophic damage in the event of timing system failure, since the loss of synchronization between the crankshaft and the camshaft will cause the valves to collide with the pistons. Warning signs that a timing chain needs to be replaced include: Later model VVT applications will generate engine codes and check engine lights prior to engine rattle Older, pre-VVT applications will generate engine rattle Aside from the belt/chain itself, also common is a failure of the tensioner, and/or the various gear and idler bearings, causing the belt/chain to derail. Furthermore, in engines where the timing belt drives the water pump, failure of the water pump can cause the pump to seize, which can break the timing belt or chain. For this reason, timing belts and chains are often sold as part of a kit with the water pump, tensioner, and idler pulleys so that these parts can be replaced to prevent timing belt or chain failure due to failure of these parts. See also Gilmer belt Interference engine Pushrod Serpentine belt References Belt drives Engine components Motor vehicle maintenance Mechanical power transmission Mechanical synchronization