Top Dead Center (TDC) for number one piston — locating

1. Top Dead Center (TDC) is the highest point in the cylinder that each piston reaches as it travels up the cylinder bore. Each piston reaches TDC on the compression stroke and again on the exhaust stroke, but TDC generally refers to piston position on the compression stroke.

2. Positioning the piston (s) at TDC is an essential part of many procedures such as timing chain/sprocket removal.

3. Before beginning this procedure, be sure to place the transmission in Neutral and apply the parking brake or block the rear wheels. Also, disable the ignition system by disconnecting the primary electrical connectors at the ignition coil packs, then remove the spark plugs (see Tune-up and routine maintenance).

4. In order to bring any piston to TDC, the crankshaft must be turned using a socket and ratchet or breaker bar attached to the crankshaft pulley bolt. When looking at the front of the engine, normal crankshaft rotation is clockwise.

5. Install a compression gauge in the no. 1 cylinder spark plug hole or place your finger partially over the number one spark plug hole and rotate the crankshaft until air pressure is felt. Air pressure at the spark plug hole indicates that the cylinder has started the compression stroke. Once the compression stroke has begun, TDC for the number one cylinder is obtained when the piston reaches the top of the cylinder on the compression stroke.

6. To bring the piston to the top of the cylinder, insert a long screwdriver into the number one spark plug hole until it touches the top of the piston.

Note: Wrap the tip of the screwdriver with tape to avoid scratching the top of the piston and the cylinder walls.

7. Use the screwdriver as a feeler gauge to tell where the top of the piston is located in the cylinder while slowly rotating the crankshaft. As the piston rises the screwdriver will be pushed out. The point at which the screwdriver stops moving outward is TDC.

Note: Always hold the screwdriver upright while the engine is being rotated so that the screwdriver will not get wedged as the piston travels upward.

8. If you go past TDC, rotate the crankshaft counterclockwise until the piston is approximately 1/2-inch below TDC, then slowly rotate the crankshaft clockwise again until TDC is reached.

9. After the number one piston has been positioned at TDC on the compression stroke, TDC for any of the remaining pistons can be located by repeating the procedure described above and following the firing order.

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