Timing 101

As the crankshaft rotates, it causes the pistons go up and down in each cylinder. As the camshaft rotates it opens and closes the valves, and also turns the distributor. This sets up a three-way timing relationship between the pistons, valves and ignition that is critical for the engine to run.

The crankshaft sprocket is half the size of the camshaft sprocket, so the crankshaft rotates twice as fast as the camshaft, causing the pistons in each cylinder to go up and down twice for each rotation of the camshaft, giving four piston ‘strokes’.

1. Intake Stroke - Piston going down, intake valve open.
2. Compression Stroke - Piston going up, intake and exhaust valves closed.
3. Power Stroke - Piston going down, pushed by the spark in the air/fuel mixture.
4. Exhaust Stroke -Piston going up, exhaust valve open.

During the intake stroke, the piston goes down drawing in a mixture of air and fuel through the intake valve. The intake valve then closes so when the piston goes back up it compresses the air/fuel mixture. At the end of the compression stroke (Top Dead Centre, TDC) the distributor sends a spark to the spark plug to ignite the compressed air/fuel mixture and push the piston back down on the power stroke. The exhaust valve then opens so when the piston goes back the burnt fuel is exhausted through the exhaust valve. This happens on all four pistons sequentially.

For the engine to run smoothly, it is designed so that each piston begins its power stroke in a specific sequence (1-3-4-2). The shape of the crankshaft determines the piston sequence and the shape of the camshaft determines the valve sequence. The ignition sequence is determined by which spark plug lead is plugged into each hole in the distributor cap as the rotor arm rotates clockwise.

The ignition timing can be advanced or retarded for all four pistons by rotating the distributor body, but the #1 piston is always used as a reference. A timing light connected to the #1 spark plug lead will flash each time the #1 spark plug fires. Pointing the timing light at timing marks on the crankshaft pulley visually indicates the relationship between piston and ignition timing.

The purpose of the timing belt is to rotate the crankshaft and camshaft together so the piston and valve timing is maintained. The teeth on the cam belt mate with the teeth on the crankshaft and camshaft sprockets. If the belt is worn it can slip and cause the valves to open on a different part of the piston stroke. The valve and piston can become damaged if the valve is fully open when the piston reaches the top of its stroke.

The cam belt should be replaced every 60,000 miles. If you buy a used vehicle, replace the cam belt immediately. The price of a cam belt is small compared to the price of a new engine, but it also must be replaced correctly!