Depending on the vehicle, those are usually the timing marks, so you can line up the belt or chain with the cams in the head(s) to the crank so the motor is on TDC, usually after you put the motor together and everything is lined up, it will fire after every part is back on the engine.
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Depending on the vehicle, those are usually the timing marks, so you can line up the belt or chain with the cams in the head(s) to the crank so the motor is on TDC, usually after you put the motor together and everything is lined up, it will fire after every part is back on the engine.