Simply put, the timing belt is a toothed belt that keeps the engine in sync, or as its name implies, in time. You can look at an engine in this manner, there are essentially 2 halves of an engine, the upper half and a lower half.
In the lower half of the engine you will find the crankshaft, and the rotating assembly which consists of the crankshaft, connecting rods, and pistons.
As the explosions happen from the fuel ignition in the cylinder, the pistons are forced downward causing the crankshaft to rotate. The crankshaft has a toothed gear on the front that the timing belt rides on. This rotation causes the belt to turn and drive the upper half of the engine that consists of the camshaft, or multiple camshafts and valves.
The camshaft(s) also have a toothed gear attached at their front end. The camshaft pushes on the valves against the springs to make them open. This lets air and fuel in on the intake side, and also lets burned exhaust gases out through the exhaust side. As you may imagine, those things must occur at exactly the right time for the engine to operate properly.
What happens if a timing belt breaks?
As the timing belt which is made out of a rubber material ages the teeth start to crack on the inside and can shear off or break. This break in the belt will allow the lower end to spin, but not spin the belt thus causing the upper end to stop moving. This is not a big problem in a clearance engine, however in an interference engine it creates a huge problem.
In an interference engine there are more than likely a valve or two that are still open when the piston comes up, and if so, will hit and bend the valve(s). This will cause engine failure and will require engine disassembly. Replacing the timing belt is a far less costly service and will prevent the engine repairs from being needed.
The timing belt also drives the water pump in a lot of engines. It is a good idea to preventively replace the pump, tensioner and idler pulleys at the same time as they are not likely to last until it is time to replace the timing belt again. Otherwise you will often be back to replace these components and pay the labor all over again doubling the expense.
How often do you change a timing belt?
The first and foremost guideline in changing a timing belt is the manufacturers recommended maintenance interval. This can usually be found in the vehicles owner’s manual or the supplemental maintenance manual. The common intervals for belt replacement are 60k, 90k, 105k or 120k miles depending on the vehicles year, make and model. Most of the newer vehicles are 105k to 120k miles but do not take that for granted. Look it up for your car or truck specifically.
Time is the other factor in belt replacement. If the vehicles mileage is low but the belt is older than 8yrs old, it is time to be replaced.
I hope this gives you a little bit better understanding of the importance of the timing belt in your engine, what it does and why it needs to be replaced.