A timing belt is a loop of a flexible yet strong material, usually a polymer of some sort, that is used in an internal combustion engine to transfer motive power from the crankshaft to the camshaft(s). For an excellent description of what a camshaft does, see Internal Combustion Engine : A Discussion of Valve Actuation Schemes by Starrynight. If the timing belt or chain in an engine breaks, you will end up with a severely damaged engine, since the valves will be left protruding into the cylinders while the pistons continue to move, resulting in shrapnel.

When mounting a timing belt (or timing chain it is critical to maintain the correct orientation between the camshaft and crankshaft sprockets. A camshaft aligned 180° out of phase will cause impressive backfires and possibly engine damage. A camshaft even a few degrees out of correct position will cause rough running and possibly engine damage.

Many (most) new timing belts come from the manufacturer with marks intended to ensure correct alignment. In the event that the belt has no mark or a used belt is being re-mounted and its marks have worn off it is advisable to make temporary marks on the old (and new) belts to aid reassembly.

The camshaft and crankshaft sprockets will have one or more registration marks to aid correct belt or chain mounting. Often but not always, these are intended to be used with the crank at TDC.

The belt tooth which aligns with the mark on the sprocket can be marked using chalk, other materials handy in some shops include lipstick or nail polish. Nail polish being semi-permanent (and requiring time to dry) is better suited to enhancing the visibility of the sprocket marks, which can be helpful when working in poor lighting conditions.

Never fold a timing belt, and avoid forcing it into a radius smaller than the smallest sprocket it passes over in use. Forcing it through too small a bend can cause delamination or damage to the fiber construction, possibly resulting in early failure.

Generally high performance engines sustain severe damage when a timing belt fails, however most modern engines are designed such that there will be no damage. As examples, a volvo 240 series just stops running when the belt breaks, while a Ducati would usually suffer serious damage to head, valves and piston.

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