An engine’s timing chain or timing belt keeps the movement of the components in the top half of an engine in synchronsation with those in the bottom half.
In the top part of the engine, the timing chain or belt is attached to the end of the camshaft, which has numerous lobes along its length.
As the camshaft spins, these lobes open and close the valves in the cylinder head to let fuel and air into the combustion chamber at the right time. The valves also allow exhaust gases to exit once the fuel and air have been compressed and ignited.
At the bottom end of the engine, the timing chain is also attached to the crankshaft, which rotates and raises and lowers the pistons at the perfect time to compress the fuel and air prior to ignition.
What is the difference between a timing belt and chain?
Timing chains largely replaced gear-driven timing systems, because they were lighter and more efficient, and then through the 1970s and 1980s they were themselves replaced in many instances by toothed timing belts, which required no lubrication where a timing chain does.
However, the timing chain has come back to prevalence over the past couple of decades because of its greater robustness and longer life expectancy.
When to replace them
Timing chains also tend to give much more warning before they fail. They stretch slightly over the course of time, which can cause the engine to run poorly or even backfire.
This is a good sign to get the chain replaced imminently. A worn timing chain will also emit a rattling noise.
Timing belts tend to simply snap, which can cause huge internal engine damage in an instant, as valves open at the wrong moment and hit the top of the rising piston.
Such damage can often write off an engine completely, hence the increasing popularity of the timing chain once more.