Top dead center for an internal combustion engine refers to the point when the piston is at the absolute top of its stroke. A piston can be a top dead center on the compression stroke and on the exhaust stroke. As a general reference point, or when installing a distributor, top dead center on the compression stroke is required. Usually it can be found by looking at timing marks but on some engines these marks may be lost or hidden. Fortunately there's an easy way to discover top dead center without the use of any timing marks.
Things You'll Need
- Ratchet and socket set
- Spark plug socket
- Plastic straw
- Breaker bar
Park the vehicle on a level, paved surface and set the emergency brake
Use a ratchet and spark plug socket to remove the spark plug from the number one cylinder.
Place a ratchet and socket on the large bolt in the center of the crankshaft. Place a finger over the spark plug hole and turn the crankshaft clockwise. When you feel pressure against your finger, the piston is coming up on the compression stroke.
Insert a plastic straw about two inches into the spark plug hole. Don't let go of the straw. Continue to slowly rotate the crankshaft clockwise. You will feel the top of the piston hit the straw and push it up. Rotate the crankshaft very slowly while holding the straw against the piston. As soon as you feel the straw start to go back down, stop.
Swap the ratchet for a breaker bar. Slowly turn the crankshaft counterclockwise. You will feel the piston come back up and start to go back down. As soon as you feel this, turn the crankshaft clockwise again. These will be very small movements and you will be able to determine when the piston is at the top of its stroke, which is top dead center.
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