Positioning No. 1 Piston at Top Dead Center (TDC)

The following procedure assumes that the spark plug wires are correctly connected and the ignition distributor is installed correctly. If it becomes necessary to bring the engine to TDC to correct the distributor position, the position of the No. 1 cylinder piston can be checked by monitoring the compression pressure change through the spark plug hole. Then align the applicable timing marks.

Top Dead Center (TDC) is the highest point of piston travel in its cylinder. Strictly speaking, in 4-stroke engines each piston reaches this position twice during crankshaft rotation: once at the end of the compression stroke and once at the end of the exhaust stroke. When adjusting valve timing and setting ignition timing, the TDC position at the end of the compression stroke (usually No. 1 cylinder) is used in the vast majority of cases.

Bringing the piston(s) to TDC is required for many procedures, in particular such as removing/installing the camshaft, timing drive components, and the ignition distributor.

Make sure the transmission is in Neutral, then firmly apply the parking brake, or chock the rear wheels of the vehicle. Also disable the ignition system by disconnecting the high-tension lead from the distributor cap and grounding it to the engine block with a jumper wire. Remove the spark plugs (see Tune-up and Maintenance section).

Bring any piston to TDC by rotating the crankshaft using one of the methods described below. When viewing the engine from the front (drive belt side), the normal direction of crankshaft rotation is clockwise:

a) The most effective method is to rotate the crankshaft in the normal direction using a ratchet socket wrench on the bolt threaded into the end of the front crankshaft journal. Remove the lower splash shield on the passenger side of the vehicle;
b) To save time, use a remote starter switch (follow the manufacturer's instructions). As soon as the piston is close enough to TDC at the end of the compression stroke, stop the starter and perform final adjustment using the method described in (a);
c) If an assistant is available, preliminary cranking of the engine can be performed in short bursts from the driver's seat under the assistant's direction. The principle is the same as described in (b). Final adjustment is again performed manually using a socket wrench (see (a)).

PROCEDURE

1. Note the position of the No. 1 cylinder spark plug wire terminal on the distributor cap.

2. If the terminal is not marked, trace the wire from the spark plug to the cap and make an appropriate mark yourself (use a scribe or marker).

3. Remove the distributor cap and set it aside (see Tune-up and Maintenance section).

4. Turn the crankshaft clockwise until the white TDC mark on the crankshaft pulley aligns with the triangular pointer on the timing belt cover.

The pulley has two timing marks. The second mark is painted red and is used only for setting the ignition timing with a timing light.
5. Now check the position of the distributor rotor. The rotor tip must point exactly at the mark on the distributor housing rim. If the rotor is 180° away from the mark, the piston of cylinder 1 is at TDC of the exhaust stroke. To bring the piston to TDC of the compression stroke, turn the crankshaft one more full turn (360°) counterclockwise. The rotor tip should now point to the mark.

6. Loosen the screw and set the inspection cover aside to gain access to the timing marks on the camshaft sprockets of both cylinder banks. Through the opening in the timing belt cover, verify that the timing mark on the camshaft sprocket is aligned with the matching mark on the rear cover.

7. When all marks (on the crankshaft pulley and the camshaft sprocket) are aligned, piston 1 can be considered to be at TDC of the compression stroke.
8. Now, to bring the remaining pistons to TDC in firing order, turn the crankshaft clockwise in 120° steps, moving from cylinder to cylinder in firing order. Make corresponding marks on the distributor housing rim (as was done above for the terminal of the No. 1 spark plug wire). Turning the crankshaft will necessarily rotate the rotor; alignment of the rotor tip with the corresponding mark confirms that the next piston is at TDC of the compression stroke.