Car distributor caps

The distributor cap as its name suggests sits on top of the distributor. It usually has a locating lug so will only fit one way.

Externally - It has a central metal contact at the top that the high voltage lead from the coil is plugged into. Then around its circumference it has 4, 6 or 8 equally spaced metal contacts. These take the spark plug leads that carry the spark to the various cylinders. Naturally a 4 cylinder engine has 4 spark plugs = 4 plug leads, a 6 cylinder has 6 spark plugs = 6 plug leads etc. It is made from a plastic or bakelite material so it is non-conductive.

Internally - The metal contacts go through the cap. The central contact usually has a spring loaded pin that permanently contacts the top of the rotor button. The rotor button rapidly spins past the metal contacts around the inside of the distributor cap and the gap to the contacts is tiny.

When a spark comes down from the coil for a particular cylinder, it comes down the central contact, along the rotor button and jumps across to the metal contact for that cylinder just as the rotor button is whizzing past it. It travels up the metal contact into the spark plug lead which routs it to the spark plug. Then it travels down the spark plug to ignite the fuel/air mix. The caps wear out over time and are a standard replacement part.

A distributor sits between the coil and the spark plugs on petrol engines. The coil provides the high voltage spark that the spark plugs use to ignite the fuel/air mix in the cylinder head combustion chamber. The distributor makes sure this spark gets to the correct spark plug at the right moment. To help facilitate this the distributor is run off the engines camshaft to get its timing. At the lower end there is a worm gear that engages with a worm gear on the engines camshaft.

This rotates the distributor shaft which runs up through the centre of the distributor. Great care is required when fitting the distributor to the engine. If installed incorrectly, the spark timing will be off and the engine will not run or could get damaged by firing a plug at the wrong time.

The rotor button sits on top of the central shaft. It is made from a non-conductive plastic or bakelite material and has a conductive brass strip that runs from the centre of it to the outside edge. As it rotates driven by the engine camshaft it distributes the spark from the coil to the edge of the distributor cap so it can be sent to the correct spark plug. It wears out over time and is a standard replacement part.

The contact or points set on a distributor causes the coil to generate the high voltage spark. Located under the distributor cap the contact set operates off a cam on the distributors shaft. The cam has a number of lobes based on the number of cylinders an engine has. A 4 cylinder has 4 lobes, 6 cylinder has 6 lobes etc.

The lobes act like an on/off switch to the coil. When on the top of the lobe the contacts open, killing the power to the coil primary winding which in turn induces a high voltage in the secondary winding creating the spark. When off the lobe the contacts are closed, recharging the primary circuit. The point sets opening can be adjusted using a feeler gauge or a dwell meter.

These items wear out the most as the contact points have a rubbing block that sits against the cam lobes. When replacing them this needs to be greased.

A distributor condensor or capacitor to give it another name sits across the points set and is mounted internally or externally on the distributor. Its prime role is to stop arcing of the points set when they open. If the condensor fails then the point set will soon burn out its contacts due to arcing, which is a spark jumping across the contacts.

Ideally replace this when changing over a contact set.