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Timing belt traps

Timing belt traps

The humble timing belt has been with us now for many years, and every so often someone gets caught out with a mismatch of timing belt and associated drive and driven gears, which almost always ends in disaster in the form of major engine damage.

There are many reasons for timing belt failure, but this is one that's often overlooked by the home mechanic along with professional technicians.

Originally in the early days of timing belts there was one tooth shape, or profile, but as technology evolved and along with higher stresses placed on belts, for example multiple camshafts and more valves per cylinder, as well as more accessories being driven by the belt. These extra demands led to the development of several more profiles in use today. The picture shows three common types, but there are more.

belt 2.jpg

A typical timing belt drive layout example could be.


Now where the mismatch occurs is when the manufacturer changes the profile of the belt during a production run. A change to the belt requires a corresponding change to every gear the inner surface of the belt contacts. The picture below shows the reason why the belt must match the teeth on the gears.

belt 1.jpg

Unfortunately this leaves an unwary technician in a position whereby he/she could say, replace a water pump with a different gear tooth profile to the rest of the gears and the belt, or maybe replace a belt with different teeth profile to the gears. It takes little imagination to figure the outcome?

It is essential to check the profile of the BELT AND ALL GEARS when any of the components are replaced. Failure to check this WILL result in rapid failure of the belt, and usually damage to the valves, and possibly pistons.
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I've had more than one timing belt break. Personal experience tells me that CD is spot on.
Once again great read! Keep them coming!