Automobiles: Timing Belt or Chain
Head gaskets have to seal the combustion chambers so very hot gases stay in, and keep coolant and oil out.
The camshaft manages the air intake for the engine by mechanically opening and closing the valves in the engine's valve stems. The camshaft gets its power from a long belt or steel chain looped around the crankshaft (coming out of, and below the engine). The camshaft is typically located above the engine, and is typically inside the engine block (to protect it and the valves connected to it from damage). In an overhead valve (OHV) engine design, valves are located in the cylinder head above the combustion chambers.
When the camshaft is below the engine, it uses "pushrods," which are long rods go up through the block and move rocker arms that in turn move the valves. The camshaft in a pushrod engine is often driven by gears or a short chain, which are generally more reliable than belt drives.
Today, most engines feature either a single overhead camshaft (SOHC) or, more likely, a double overhead camshaft (DOHC). With DOHCs, sometimes called a "twin cam," two overhead camshafts are used, one for the intake valves and second for the exhaust valves. In a V6 or V8 engine, two SOHCs or DOHCs are used, one for each bank of cylinders.
More valves create a smoother flow of engine intake and exhaust gases freely because there are more openings, creating more explosive power from the same engine size (or "displacement"). Engines with more than four valves per cylinder use DOHCs because there are too many cam lobes for a single camshaft.
The toothed timing belts on SOHC and DOHC engines need to be replaced periodically. If a belt breaks, or if it loses of some of its teeth or ratchets, the camshaft will stop turning and result in serious engine damage.
There are two types of overhead camshaft designs, free-running or interference. The free-running, or free-wheeling, cam has enough clearance between the valve and piston that they never touch, even if "out of synch". In contrast, the interference camshaft closely synchronizes the valve and piston through overlapping cycles. If the free-running camshaft stopped working, the engine performance drops with only a slightly open valve, but with interference camshaft, serious piston damage (if not engine damage) can occur. Today's high compression, emission-controlled are more likely to be of the interference type.
Rather than risk being stranded on the side of the road, or worse, end up with an expensive engine repair bill, you should heed the advice of the auto manufacturers. Timing belts should be replaced at the time or mileage interval specified in your owner's manuals, typically 60,000, 80,000 or 100,000 miles. This recommendation is based on how long before such belts typically fail. The most practical time to check the belt is during a tune-up, when the spark plugs are out and the engine can be rotated quite easily. Unless, you are an experienced mechanic, the inspection and replacement of belts should be left to the professionals.
Dirt, coolant and oil inside the timing belt cover can increase the chances for belt failure, so inspection and if required, replacement, should be done as soon as possible. And you should find & fix the source of the leak.
Chain camshaft drives for more expensive and high performance cars usually last the life of the engine, but are more expensive. Because they are narrower than timing belts, manufacturers like using them in compact engine configurations. They do, however, need periodic adjustment as per your Owner's Manual. Typically, the chain will start getting noisy long before any damage to the engine begins.
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