TIMINGBELT

In the internal combustion engine application, the timing belt connects the crankshaft to the camshaft(s) which in turn
controls the opening and closing of the engine's valves. A four-stroke engine requires that the valves open and close
once every other turn of the crankshaft. The timing belt does this. It has custom teeth to turn the camshaft(s)
synchronized with the crankshaft and is specifically designed for a particular engine. In some engine designs, the
timing belt may also be used to drive other engine components such as the water pump and oil pump.

Timing belts are typically inaccessible and difficult to inspect. Replacement at specific intervals is recommended by
the manufacturer. The manufacturer may also recommend the replacement of other parts, such as the water pump,
when the timing belt is replaced because the additional cost to replace the water pump is negligible compared to the
cost of accessing the timing belt. Failure of the timing belt will leave the engine non-functioning. Depending on the
design of the engine, the piston and valve paths may "interfere" with one another and incorrect timing in their
movements may result in the piston and valves colliding. (Such designs are also called "interference head" or
"interference engines". Conversely, non-interfering engines are called "free-wheeling" or "non-interference" engines.)

In interference designs, regular service is especially important as incorrect timing may result in the pistons and valves
colliding and causing extensive engine damage and therefore costly repairs. The piston will likely bend the valves or if
a piece of valve or piston is broken off within the cylinder, the broken piece will cause severe damage within the
cylinder, often also affecting the crankshaft. In some newer engines, timing belts are designed to last the effective life
of the engine. When a timing belt is replaced, care must be taken to ensure that the valve and piston movements are
correctly synchronized.

A timing belt is typically rubber with high-tensile fibers running the length of the belt. Rubber degrades with higher
temperatures and with contact with motor oil and antifreeze. Thus the life expectancy of a timing belt is lowered in hot
or leaky engines. Newer or more expensive belts are made of temperature resistant materials such as
"highly-saturated nitrile" (HSN). Older belts have trapezoid shaped teeth. Newer manufacturing techniques allow for
curved teeth that are quieter and last longer.