Unlike modern cars with their fancy, set-and-forget electronic ignition systems, the majority of classic cars are fitted with distributors and contact points, giving us all something to tinker with on ‘garage’ days — and something to go wrong if neglected.
However, before we look at fitting and gapping points, let’s run through the way a ‘classic’ ignition system works.
The essential parts of the ignition system are the battery, ignition switch, coil, distributor, condenser, contact breaker, rotor and, of course, the spark plugs. When you turn the ignition key, low tension current from the battery (at either six or 12 volts) is transformed by the coil into as much as 80 to 10,000 volts, that current is then passed by the distributor to produce a spark for each engine cylinder. The flow of current through the coil produces a magnetic field as long as the contact-breakers (or points) remain closed.
When the contact-breaker cam is struck by the distributor shaft, the breaker opens and the magnetic field collapses and induces a secondary high tension current in the secondary coil. This then travels to the distributor cap, where it is picked up by the rotor via a small, spring-loaded carbon brush. The spinning rotor then distributes the charge to whatever terminal is closest to the rotor in the cap — each terminal, of course, leading to a spark plug.
The condenser manages everything by absorbing whatever currents are induced in the primary circuit at the moment when the circuit breaks. Take away the condenser and the points would arc, leading to premature failure.
To give you an idea of how hard this type of ignition circuit works; a single spark plug produces a minute spark (typically around 0.5mm), at 80 to 10,000 volts 20 to 40 times a second at normal cruising speeds.
Fitting and Re-setting
¢ Distributor contact-breakers consist of two metal points — one fixed and one spring-loaded so that it can open and close as the cam in the distributor spins around. The movable point is insulated so it can only earth through the contact face and complete the circuit.
¢ Contact points should be checked every 10,000km — sooner if you suspect a problem. If the points are burned or pitted don’t be tempted to simply file the faces. That’s okay for a roadside repair, but a new set of contact points should now be fitted.
¢ Many classic cars would’ve originally come with a two-piece contact set, but modern replacement are usually of the easier to fit one-piece type. Here’s how you fit and set a more modern one-piece contact set. (Although most classic car’s distributor types vary in size, shape and brand depending on your specific marque, the following process should apply to most.)
Disconnect the battery first.
¢ Unclip the distributor cap and pull off the rotor. (It’s not a bad idea to check the contact points inside the cap and rotor tip for wear or pitting, or even better, replace both when installing new contact points.)
¢ Use a spanner to remove the nut securing the old contact set to the distributor base-plate.
¢ Disconnect the low-tension lead and condenser lead. (You should also consider replacing the condenser at this point.)
¢ Wipe the new contact points clean to remove any dirt or grease.
¢ Remove the nut on the plastic bush and position the new contact set onto the base-plate.
¢ Replace the clamping screw with its washers in the correct order.
¢ Reconanect the low tension and condenser leads.
¢ Tighten the nut onto the plastic bush — not too tightly. Make sure that the two lead-tags make good contact or misfiring will occur.
Now it’s time to set the points:
¢ With the ignition off and with the distributor cap and rotor removed, turn the engine by hand until the shoe of the moving point is on the peak of one cam.
¢ The points should now be fully open and can be checked with a feeler gauge.
¢ Check your owners’ manual to determine the correct points gap.
¢ Loosen the securing screw and move the contact-breaker base-plate with a screwdriver until the points just touch a feeler gauge inserted between them.
¢ Once the correct gap is set, tighten up the screw.
¢ Re-check the gap — if it has moved, repeat steps four to five again — and re-check again.
¢ Lightly oil the pivot post, making sure you don’t drop oil onto the point. Most point sets comes with a small sachet of light grease, which should be smeared onto the cam.
¢ Refit the rotor and distributor cap.
If you did everything correctly, your engine should now be running smoothly when you take it out for a test-drive.
By James Black
This article is from Classic Car issue 239. Click here to check it out.