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Behold Your Timing Belt

Technicians sometimes refer to the camshaft as the brain of your engine because it keeps all those pumping valve train parts working together so the air/fuel mixture enters and exits the cylinders on cue. If that were the case, perhaps a better analogy would be stage manager. Still, there's something else behind the scenes of this automotive drama that helps run the show, yet looks far less impressive than those finely ground cam lobes. It's your timing belt (or chain), that vital link between the rotation of the crankshaft and that of the camshaft. Knowing what it does and how to take care of it can save you hundreds on engine repair.

In earlier engines, camshafts were often gear-driven off the crankshaft. Later on, engine designers developed chain drives in OHV (overhead valve) configurations that allowed some flexibility in the placement of the camshaft so that shorter pushrods could be used, all for more performance and efficiency. Those engines with long chains sometimes tended to whip about and cause problems. The only alternative was a noisy and complicated multi-gear train until the cogged rubber synchronous timing belt was invented in 1945. Though it was once considered the hallmark of a cheap engine, now it is used in distinguished automobiles such as Volvos and Porsches.

Each type of camshaft drive has pros and cons. Gears are dependable but noisy, and the cam must be placed near the crankshaft to keep the sprockets reasonable in size. Chains, either the roller or "silent tooth" variety, are quiet and convenient, yet can stretch over time, changing valve timing or even jumping a tooth. Long chains require a tensioner or stabilizer that will wear over time. In addition, the nylon teeth used on many cam sprockets have a reputation for disintegrating, which can result in bent valves (unless the engine is a "freewheeling" type where the pistons and valves can't come into contact with each other).

Rubber belts are quiet, inexpensive, stretch very little in use, and can be replaced without violating the oil tightness of the engine. The big drawback is that they can snap without warning, and if the engine is not of the free-wheeling type, the pistons will probably bend the valves, requiring cylinder head removal and an expensive replacement of valve train components. That's why you need to know when to replace this unsung hero of your engine.

Vehicle manufacturers usually indicate regular replacement intervals in order to avoid this catastrophe. Don't put it off! It's a perfect example of the old sayings, "A stitch in time..." or "An ounce of prevention..." Use whatever aphorism you choose, just make sure you replace the belt when recommended. Many timing belt replacement intervals have been increased because belts last longer than they once did, partly due to improved belt tooth design. Older cars generally use the trapezoidal-shaped tooth belts, while the rounded, or curvilinear, tooth design was introduced around 1980. The curved tooth design creates less friction and therefore runs cooler. Modified Curvilinear, or HTD II, came out around 1985 and is supposed to be the most durable design. The sprockets for these belts are different, and using the wrong application belt will result in rapid failure.

Experienced mechanics find that many headaches can be avoided by replacing the timing belt tensioner with every timing belt job. Serious consideration should be given to replacement of timing belt-driven water pumps, or water pumps that are trapped behind the timing belt.

Out of all these points, the simple thing to remember is this: follow the recommended service interval for belt replacement. If you don't, you may end up suffering from a case of really bad timing.