by Nigel Holmes

Created on: January 03, 2010

The engine in every car or truck uses either a timing chain or belt to connect the crankshaft to the camshaft. This turns the camshaft at half the speed of the engine and ensures that the inlet and exhaust valves open at the right time in relation to the movement of each piston in its cylinder bore.

Timing chains are largely maintenance-free and will generally last the life of the engine. Unfortunately the same is not true for timing belts. These are complex constructions of fiber-reinforced rubber with teeth molded into the inner surface. The teeth engage with matching notches on the crank and camshaft pulleys, ensuring that the rotation of two shafts is synchronized.

If the belt fails the camshaft will stop turning and the valves will neither open nor close. However, the inertia of the flywheel will keep the pistons moving up and down in the cylinders for several more revolutions of the crank. In some engines this will result in the pistons hitting the valves. When this happens the valves will bend and the pistons may crack. The result is that the engine has to be torn down and completely rebuilt.

This disaster can be avoided if the timing belt is replaced periodically. However, as there is no way to inspect the condition of the belt this must done as specified by the manufacturer. Intervals vary depending on the design of the engine and the precise construction of the belt used, but for most vehicles the recommendation is somewhere 60,000 and 90,000 miles. (Check the handbook or call the dealer to get the actual figure for your car or truck.)

If you purchased your car new you’ll know when to get the belt changed, but if you buy used its not so clear-cut. Timing belt replacement is an expensive job so many owners decide to sell rather than have the work done. For this reason, when you go to buy a used vehicle always ask if the belt has been changed. If you’re told it has, ask to see the invoice for the work or the service history. If there’s no evidence of the work being done, and the vehicle has done over 60,000 miles, it’s prudent to book it into the dealer for a belt replacement. The bill for this job, typically around $500, might sting but it’s one tenth of the cost of a new engine, so think of it as insurance.

Given the consequences of belt failure, you may wonder why car manufacturers don’t just fit timing chains. The official reason is noise: chains will inevitably slap and rattle, especially when cold, and buyers expect engines to be silent. The other reason is of course cost. Chain systems are more complicated and need lubrication, which adds yet more cost. As new car buyers are generally rather price-sensitive, the manufacturers prefer to make belt replacement part of the cost of ownership.

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