A Distributor is a combination of two mechanically activated switches in a plastic and metal housing. One of the switches is a simple on and off switch. This is called the points.

The other switch is a rotary switch that rotates to as many contacts as you have cylinders in your engine. The moving part of the switch is called the rotor. The switch contacts are in the distributor cap. The rotor sits on top of the distributor shaft and is rotated by the shaft so that it touches each contact in succession.

Just below the rotor is a multi-lobed cam that opens and closes the points. There are as many lobes on the cam as there are cylinders in the engine.

Points (the on-off switch)

The points are a replaceable on and off switch located on a plate within the top of your distributor. The points control when the coil will send a jolt of electricity through the cap and rotor to a spark plug. The spark occurs as the points just open.

The points have two contacts. One contact is stationary. The other contact has a plastic follower on its arm that rides on the distributor shaft cam. The cam pushes the follower, opening and closing the points.

The stationary point is grounded, providing a return path to the battery. The moving point carries electricity from the low voltage side of the coil. When the points are open, high voltage flows from the coil, through the center contact in the distributor cap, through the rotor, the cap, the spark plug and to the ground.

When you set the point gap, you are setting up the length of time the switch will be on & off in relation to the turning of the distributor shaft. 

Cap & rotor (the rotary switch)

The rotor sits on top of the distributor shaft and rotates around in circles. It has a metal strip that goes from the center of the rotor top to the outside of the rotor cap. This is a switch contact. The cap has a center contact and as many contacts around the top edge of the cap as there are cylinders in the engine.

The center contact is a spring loaded carbon contact that rides on the center of the rotor. The Rotor is just long enough to reach the outer contacts on the distributor cap. The high voltage electricity from the coil goes through the big wire coming out the end of the coil to the center of the distributor cap. It then travels down the carbon contact to the metal plate on the rotor, along the metal plate to one of the outer contacts on the top of the distributor cap. It goes out of the distributor, through the spark plug wire to the spark plug. A spark jumps the gap in the spark plug igniting the air fuel mixture causing an explosion in the cylinder.

The important thing is that the rotor needs to be pointing at the right terminal when the jolt of electricity is coming from the coil. This is set by rotating the distributor body.

To set the point gap:

Locate the plastic cam follower on the movable point arm. To set the timing, this follower needs to be sitting at the tip of a cam lobe. If you are going to set the timing afterwards, you can loosen the ring at the base of the distributor and rotate the distributor until the plastic follower is sitting on the tip of a cam lobe. If you are not planing to adjust the timing afterward, put the car into fourth gear and push the car forward until the plastic follower is sitting on the tip of the cam lobe. This is best done with a helper and in a place where the car will not get away from you. Alternatively, you can put the transmission into neutral and turn the engine over with the starter crank or use the starter handle to turn the engine.

Once the plastic follower is sitting at the tip of the cam lobe, you can adjust the point gap. On most British cars, including Series Land Rovers,the gap is set to 15 thousandths of an inch. You slightly loosen the screw holding the points down and move the arm until the gap is correct.

This means the 15 thousandths feeler gauge will go between the points without opening them farther, but a 17 thousandths feeler gauge will not. Tighten the mounting screw. You are now ready to adjust the timing.

Continuity Light:

A continuity light is used to adjust the timing when the engine is not on. The process is called stationary timing.

A continuity light can be purchased at any auto parts store or you can make one yourself. It is basically a light with two wires attached. You connect one wire to ground and the other to the movable arm of the points. Turn the ignition switch to the on position. When the points are open, the light is lit. When the points are closed, the light is not lit.

Adjusting the timing:

The distributor shaft rotates in a counter clockwise direction. If you turn the distributor housing counter clockwise, the direction that the rotor is turning, you are retarding the timing. If you turn the distributor clockwise, into the rotation of the rotor you are advancing the timing.

First, locate the number one spark plug. For inline engines, it is the one closest to the radiator. Follow the number one plug's wire back to the rotor. When the number one plug fires, the rotor will be pointing to that contact . Remember where the contact is located along the circumference of the distributor then remove the cap so you can see where the rotor is pointing.

Line up the timing marks with the rotor pointing at the cap terminal that has the spark plug wire going to the number one spark plug. Since the crank rotates twice as fast as the distributor shaft you have a 50% chance of getting it right the first time. If the rotor is pointing to a different terminal, rotate the engine one more time until the timing marks are lined up and the rotor is pointing to the number one plug.

What you want is for the points to just be opening when the timing point is lined up. This is where the continuity light just comes on.

If you look at the plastic follower that rubs against the distributor cam, you want it to be on the leading side of a cam lobe. This will be the counter clockwise, or right side of the cam lobe.    When finding the point where the points just open it is important that the distributor shaft be turning in the direction it normally travels when the engine is running ( turn the distributor  clockwise  not counter clockwise).

Attach the continuity light and turn the ignition switch on. With the follower on the correct side of the lobe, slowly turn the distributor clockwise until the continuity light just comes on (The points just start to open). If it's already on, move the distributor body counter clockwise (retard) until the continuity light goes off. Then slowly rotate it clockwise. If you go past that point, back the distributor up then try it again by turning the distributor clockwise.  The timing will be off if you try to reach the place where the light just comes on by backing up.

Once you are satisfied that you have the distributor body to the point where the points are just opening (the light just goes on), turn the ignition off and tighten the base of the distributor. Reassemble everything and go out for a drive.

There is now an FAQ page on trouble shooting the ignition system. It goes into the mechanical and vacuum advances.

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