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How to Find Top Dead Center

Posted in How To on March 1, 2002 Comment (0)
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How to Find Top Dead Center
Photographers: Cole Quinnell
Install an indicator or pointer somewhere on the block to serve as a reference point to the degree wheel. Next, rotate the crank until the No. 1 piston is close to the top of the cylinder. Adjust the degree wheel or the indicator to the zero mark on the wheel. If you’re using a plate-type stop, be sure to place the stop toward the middle of the piston, because the piston may rock in its cylinder, throwing the calibration off slightly. Install an indicator or pointer somewhere on the block to serve as a reference point to the degree wheel. Next, rotate the crank until the No. 1 piston is close to the top of the cylinder. Adjust the degree wheel or the indicator to the zero mark on the wheel. If you’re using a plate-type stop, be sure to place the stop toward the middle of the piston, because the piston may rock in its cylinder, throwing the calibration off slightly.
Rotate the crankshaft until the piston is halfway down the cylinder, then install the piston stop.  Now gently rotate the crankshaft until it hits the stop. Record the number of degrees indicated on the degree wheel. Then rotate the crankshaft in the opposite direction until the piston hits the stop again, and record that number. Top dead center (TDC) is exactly halfway between these two marks, so count the number of degrees between the two marks and divide by two. Rotate the crankshaft until the piston is halfway down the cylinder, then install the piston stop. Now gently rotate the crankshaft until it hits the stop. Record the number of degrees indicated on the degree wheel. Then rotate the crankshaft in the opposite direction until the piston hits the stop again, and record that number. Top dead center (TDC) is exactly halfway between these two marks, so count the number of degrees between the two marks and divide by two.
In this example, the first number is 17 degrees after top dead center (ATDC), and the second number is 29 degrees before top dead center (BTDC). The total number of degrees between them is 46. Divide this by two to get 23. Now, without moving the degree wheel or the crankshaft, move the pointer so that it is 23 degrees from TDC. In this case, it would be 23 degrees BTDC. Double-check your measurements by repeating the previous steps. The numbers should be identical. Remove the piston stop and rotate the crankshaft and degree wheel together until the TDC mark lines up with the pointer. Your No. 1 piston is now at top dead center. In this example, the first number is 17 degrees after top dead center (ATDC), and the second number is 29 degrees before top dead center (BTDC). The total number of degrees between them is 46. Divide this by two to get 23. Now, without moving the degree wheel or the crankshaft, move the pointer so that it is 23 degrees from TDC. In this case, it would be 23 degrees BTDC. Double-check your measurements by repeating the previous steps. The numbers should be identical. Remove the piston stop and rotate the crankshaft and degree wheel together until the TDC mark lines up with the pointer. Your No. 1 piston is now at top dead center.
With the harmonic balancer carefully reinstalled, mark TDC with a fine brush and paint,   or use something equally as permanent. With the harmonic balancer carefully reinstalled, mark TDC with a fine brush and paint, or use something equally as permanent.

Finding top dead center (TDC) can be a bit more tricky than most people might think. It is impossible to accurately tune an engine without finding TDC precisely, because it is the primary reference point on which you base piston-to-valve clearance, ignition timing and cam timing. In other words, finding TDC is vital.

Finding true TDC is a fairly straightforward operation, but it does contain some nuances and subtleties that can be confusing. To accurately find it, you’ll need a few tools: a degree wheel, a piston stop, an indicator and a crankshaft nut. This procedure can be done with the heads on or off, but the type of piston stop you’ll need depends on whether or not the heads are installed on the engine. A screw-in-type stop threads into the spark-plug hole, while the other style attaches to the deck surface of the engine block.

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