Timing Belts

Most people have heard of them, some people have experienced it when they break, and the unlucky ones of those have felt the strain on the wallet when severe engine damage occurs.

Q: So what exactly is a timing belt and why do we have them?

A: A timing belt is a rubber belt with teeth that connects the camshaft to the crankshaft. These belts are used on "overhead cam" engines because they are inexpensive to make and are quiet when they run.

Q: So why do some engines experience severe damage when a belt fails and others do not?

A: Engines that use timing belts are classified into two categories - interference and non-interference.

The first category - interference, is the group of engines that experience damage when the belt breaks. To understand why this happens, you must first understand how an engine works. The very abbreviated version of an engines operation is this. When the crankshaft turns, the pistons move up and down. When the camshaft turns, it moves the valves up and down. The timing belt connects the crankshaft to the camshaft and controls the "valve timing". The "valve timing" is what allows the valves to go up and down or open and closed at the proper time to make the engine run. The valves are in the up or closed position when the pistons are at the top of their travel. When a timing belt fails the camshaft stops turning, at least half of the valves are open or in the down position and they stay there. The crankshaft, however, continues to turn moving the pistons up and down. How many revolutions the crankshaft makes after the belt breaks varies depending on many factors. The "interference" engine allows the pistons to hit the open valves. The least damage that will occur is to bend the open valves causing them to not seal and give the engine no compression. Without compression, the engine will not run even after the faulty belt is replaced. The next step is to remove the cylinder head(s) and replace the bent valves. In worse cases, the pistons break when they hit the valves and in severe cases the cylinder walls can crack - Ouch!! Talk about expensive!

The second type of engine is non-interference, what this means is that even when the valves are wide open, down, and the pistons go up, they do not hit the valves. No damage occurs in this type of engine.

Q: How do I know what type of engine I have?

A: Most auto parts stores or auto repair facilities have access to this information. It would be a good idea to know that type of engine you have so you have enough information to determine how much you want to gamble on when to replace your belt.

As nothing is carved in stone when it comes to auto repair, we have seen "interference" engines not bend valves when the belt breaks and we have seen "non-interference" engines bend valves when the belt fails. Go figure.

Q: How often should I replace my timing belt?

A: Replacement intervals vary from engine to engine; this is due to many things. Some belts are narrow, some are wide. Wide belts last longer than narrow ones, some belts have square teeth and some have round teeth. Round teeth last longer as they kind of roll in and out of the gears. Types of belts are not interchangeable so we can't put a wide, round tooth belt on an engine equipped with a narrow square tooth belt. The owner's manual has the suggested mileage interval to replace the belt. Please take in mind it is a suggestion. We have seen belts fail early while others exceed their expected life. If you do not have an owner's manual, we at the CARdoctors® of the Antelope Valley would be happy to look in our extensive computerized data base to find your suggested interval.

Q: Is there ever anything else involved when replacing a timing belt?

A: Yes, quite often there is more involved. There are idler pulleys and tensioners and tensioner pulleys that can fail causing premature belt failure. If these items are not replaced, you take a chance on premature failure. Some engines have water pumps that are driven by the timing belt. If it is noisy or shows signs of leakage, it would be wise to change it. You guessed it - it can cause premature failure! There are oil seals at the front of the motor. If they leak, oil will get on the belt and cause premature failure. Oh, there's one other thing - how are your other drive belts? They probably need changing also - might as well do them while they are off.


When a timing belt job is done the right way and done before it breaks, it will help ensure you have a dependable vehicle. It will also save you valuable time and money in the long run.

Call the CARdoctors® of the Antelope Valley at (661) 949-8484 with any questions and don't forget to set your appointment. Don't be one of those people who spend thousands to repair when they could have spent hundreds to prevent!