Be it an Acura or a Yugo, a failed timing belt is not a pretty sight.

The engine, of course, will stop if the timing belt breaks, gets stripped of enough teeth or ratchets excessively.

But the real danger lies not in the inconvenience of being stranded on the road. It is in the real possibility of severe engine damage, warn engineers from The Gates Rubber Company.

Automobile engines can be classified as either free-running or interference, depending on what occurs if the piston-to-valve synchronization is lost due to a failed timing belt.

In the free-running (free-wheeling) engine with the crankshaft still moving, there is still enough clearance between the valve and piston, even if the cam stops with a valve fully open.

However, an interference engine may allow the piston and the valve to collide. The least damage could be a bent valve. The most damage may be a hole in the piston, or damage to the head, or gouged cylinder heads, or worse yet, a completely ruined engine.

Most domestic cars are free-running. Several notable exceptions are some models of the Chevy Spectrum and Chevette, Plymouth Colt, and the Pontiac LeMans and Sunbird. (The majority of these models have foreign-made engines.)

Most import cars, however, have higher compression engines, and are most likely to be the interference type. More than a dozen manufacturers, from Acura to Yugo, have produced this type of engine.

Also, all diesel engines are interference. A compression ratio of 20:1 doesn't leave much combustion chamber space to spare.

Rather than risk an expensive engine repair, Gates suggests car owners heed the advice of the auto manufacturers and most independent repair technicians.

The auto makers say that timing belts should be replaced at the time or mileage interval specified in their owner's manuals, regardless of the physical appearance of the belt.

Independent technicians agree with the OE's, but to be safe, they also recommended that the belt be inspected between 30,000 and 50,000 miles.

This inspection process can vary depending on the engine placement, and the number and location of engine accessories.

The most practical time to check the belt is during a tune-up, while the spark plugs are out and there is no engine compression.

A simple inspection may involve rotating the drive and feeling for slack or play in the belt. This may indicate a worn or stretched belt, or a loose tensioner. On some engines, the belt guard can be partially pulled away for a quick visual inspection.

The most thorough inspection, however, involves removing the belt cover to check every tooth and land (the flat area between the teeth). Care should be taken when handling the belt, though. Do not twist the belt more than 90o, or the tensile cords could be damaged.

Look carefully at the sides and top of the timing belt. Watch for worn or frayed fabric tooth facing, glass fibers protruding through the belt, tooth cracks, scratches or grooves on the back of the belt, or oil on the belt or sprockets.

A timing belt can have hidden damage. Gates explains that sometimes the tensile cords can fail due to an engine malfunction resulting in a shock load to the belt. Cords can also be damaged by a foreign object as small as a pebble coming between the belt and sprockets.

Also, the integrity of the belt environment is especially important. If dirt, grease and oil are permitted within the timing belt cover, the chances for belt failure are greatly increased.

Especially if it's a valve-crunching interference engine, it shouldn't be too hard to convince anyone to check the belt, and replace it at the first signs of fatigue.

If you wait for the belt to break, you'll be sadder and wiser--but poorer.

Provided as a public service by the Gates Rubber Company