To find top dead center in an internal combustion engine, it is necessary to stop the movement of a piston. Typically, most engine builders will choose to find top dead center with the No. 1 cylinder. Using a dial indicator gauge and a crankshaft socket, it is possible to find the top dead center of a piston's stroke by taking a reading on the dial indicator while turning the rotating assembly. Once located, it is much easier to find top dead center by using a piston stop bolted to the engine block and adjusted to just come into contact with the top of the piston when it's at the very top of the stroke.
It is very important to find top dead center of an engine's stroke in order to set the ignition timing. All ignition timing is set and adjusted according to the piston's relation to top dead center of the crankshaft's stroke. Once an engine is running, the ignition timing can be set using a timing light and the reading from the harmonic balancer. Initial start up requires the engine builder to find top dead center and set the timing to fire at that point in the engine's rotation. All final adjustments can be made once the engine has been started.
If an engine is mounted in a vehicle and the cylinder heads are in place, it is still possible to find top dead center. After removing the spark plug wires to disable the engine and prevent accidental start up, the No. 1 spark plug must be removed. Once removed, a small screwdriver can be inserted into the spark plug hole, and the engine can be rotated by turning the crankshaft pulley. This can be done by hand or by bumping the starter. While the engine is rotating, it is necessary to hold onto the screwdriver and feel when it contacts the piston to find top dead center.
When the piston contacts the piston stop, it will begin to be pushed up and out of the spark plug hole. At the point that the screwdriver ceases to be pushed up and out of the cylinder, the engine should be gently rocked back and forth by hand. When it feels like the screwdriver is neither going up or down, the search to find top dead center has been completed.
With the engine in this position, the spark plug can be reinstalled, the plug wires can be reattached and the ignition rotor can be positioned so it is pointing to the No. 1 spark plug wire. The vehicle should then start and run. Now, the ignition timing can be set with a timing light.