Mechanics sometimes refer to the camshaft as the brain of your engine because it keeps all those pumping valvetrain 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 a bundle on engine repair.
Revolution Evolution
In any four-stroke Otto cycle engine (in contrast with the rare Miller cycle type found on the Mazda Millenia S sedan), the camshaft turns at half the speed of the crankshaft. In other words, the crankshaft must make two revolutions in order to turn the cam one revolution. Connecting the rotation of these two shafts so they operate in unison is the timing belt or chain, which on some engines may turn a water pump as well.
In earlier engines, camshafts were often gear-driven off the crankshaft. Later on, powerplant 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 Acuras, 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 cogs 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 head removal and an expensive replacement of valvetrain components. That's why you need to know when to replace this unsung hero of your engine.
Critical Care
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. Manufacturers' service intervals for timing belts can vary from 10,000 miles for a simple belt tension adjustment to well over 100,000 miles for replacement.
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.
Speaking of running cooler, heat is the big enemy of a timing belt. If you're operating a vehicle at high speed in hot areas, time may be just as important a factor as mileage in the durability of the belt. Oil and antifreeze can also attack timing belt rubber. That little oil drip out the front of the cover may not look like much but could be shortening your timing belt's life.
Other reasons for premature failure include improper belt tension during replacement. Over-tightened belts run noisy and create additional heat. Sloppy workmanship on installation of a timing belt cover can also damage a belt.
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.
