Replacing a timing belt can be a complicated, time-consuming operation. Depending on the application, the process often involves removing motor mounts, valve covers and parts such as the alternator and power steering pump. Additionally, care must be taken to ensure that the replacement belt is installed in such a way that the camshaft and crankshaft remain properly synchronized. Failure to do so can result in anything from a poorly running engine to catastrophic engine damage.
The timing belt itself is an important part of any engine that makes use of one. Its purpose is to keep the camshaft and crankshaft synchronized, so the valve train operates in time with the pistons. The exact replacement process will differ greatly depending on the make and model of the vehicle but, generally, everything attached to the front of the engine has to be removed or moved out of the way. This generally involves removing the drive belts, accessories and associated brackets, motor mounts and the crankshaft pulley. Once all of these components are removed, it becomes possible to access the timing cover.
A timing cover is a plastic or metal cover that is designed to protect the timing belt. Once removed, the timing belt itself may be accessed. To facilitate correct reassembly, most applications have a specific timing belt removal procedure. This often involves turning the crankshaft either clockwise or counter-clockwise to remove belt slack and lining up marks on the cam and crank gears. Tensioner and idler pulleys are often replaced at this time and, in applications where the water pump is driven by the timing belt, it is often replaced, as well.
When a new belt is installed it is important that the crank and cam gears are in the same orientations as they were prior to the removal of the old belt. The new belt can then be tensioned, the process for which can differ depending on whether the application uses an automatic or manual tensioner. Manual tensioners must be set by prying the tensioner and measuring the belt tension, while an automatic tensioner need only be released from its compressed state to apply the correct tension.
If the timing belt broke prior to replacement, the engine can be compression tested at this point. Such a test can reveal whether damage was done to the valves when the belt broke. This sort of valve damage can also occur if the timing belt was incorrectly installed, which is why it's so important to ensure that the cam and crank gears are correctly synchronized when installing the new belt.