Timing belt: It transmits movements from the crankshaft to the camshaft. It keeps the engine synchronized.
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Serrated belt located in a precisely determined position that synchronizes the movements of the crankshaft with the movements of the camshaft so that it opens and closes the engine's valves when the timing is right. If it is not correctly attached, the cylinder head and the pistons may get seriously damaged.
I threw a timing belt and re-valved the heads, but how to re-set timing with due overhead cams? I see timing marks, but which cylinder should be at top dead center?