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Magnetos

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The magneto was fitted to most cars of the pre-war years – and is still a requirement for modern piston-engined aircraft

Magnetos date back to the dawn of the internal combustion engine. From the early low-tension magnetos that developed fewer than 1000 volts and used igniters to fire the engine were developed high-tension magnetos, generating 8000 to 20,000 volts to fire spark plugs.

We’re familiar with magnetos on vintage cars and motorcycles, but they’re also to be found in our garden sheds, being the ignition system of choice for lawnmowers because of their simplicity, cheapness and ability to work without a battery. Similarly, they’re used in aircraft – piston-engined aircraft are required to have dual magneto systems as it is usually possible to anticipate failure with magnetos, whereas electronic systems can fail without warning. They’re even used on top fuel dragsters, because modern magnetos pack a bigger punch than most other ignition systems.

A magneto is basically an electromagnet in reverse: instead of putting a current through a coil of wire to create a magnet, you spin the coil between the poles of a U-shaped magnet, thereby generating an electrical current in the primary coil. There are also other types where it is the magnet that rotates, or polar inductor magnetos with fixed magnets and a rotating magnetic chopper.

A set of contact points is used to break the flow of current through the primary coil just as the magnetic field reaches its maximum, creating a voltage spike. By using a primary coil of relatively few turns of thick wire and a secondary coil of many turns of thin wire, the voltage is boosted in the secondary coil to around 20,000 volts, which is fed to the spark plug.

The voltage is at its highest at only two points in each revolution of the primary coil (armature), so precise gearing of the magneto to the engine is essential: you cannot drive a magneto by belt or any means that might slip and you must gear it up or down according to the number of cylinders the engine has. A four-stroke, four-cylinder engine has two firing strokes per engine revolution, so the magneto can be driven at engine speed.

There’s not a great deal that you can safely do to a magneto yourself: even dismantling one may be unwise, as special tools are needed and some magnetos have very fragile insulators inside that must be disconnected before you pull the unit apart. If you don’t know they’re there, you will cause expensive damage (and replacement parts, costly as they are, may not be as good as the originals). You also have to remagnetise it every time you dismantle it!

Changing the bearings also requires special tools, otherwise they can be damaged: the casing has to be heated to 120˚C and the bearings put in hot with an insulator; then, once cooled, they must be shimmed on the shaft to a specified preload, otherwise the armature or the bearing will be wrecked.

What you can and should do is keep them clean. Wipe the slip ring and pick-up regularly and keep the points clean, free of oil and petrol and correctly adjusted. The bearings need lubrication, but it’s just as important not to over-oil them.

Magnetos have always been a black art, and those who really know how to rebuild them tend to keep the trade secrets to themselves – and the best repairers are quite elusive, not needing to advertise because they’re always snowed under with work. Tony Stairs has over 40 years’ experience and is particularly sought after for aircraft magneto work: you don’t want a sudden failure with an aircraft magneto. Choose your specialist by personal recommendation: some have set up in the past with minimal experience and very high failure rates.

‘Magnetos lasted 50-60 years originally,’ Tony explains, ‘and you should get at least 20 years out of a properly rebuilt one – but they’re not all getting that.’

Lucas magnetos are a particular problem, because they used Bakelite for some parts and, as it gets old, it can get electrically leaky when it’s hot. If those parts are not replaced when the magneto is rebuilt, the engine may display a mysterious misfire and power loss, especially over 2500rpm. Tony has heard of engines being stripped at great expense because the ‘rebuilt’ magneto was never suspected.

Replacement Bakelite parts are not readily available and finding an alternative that can stand the heat and the friction from the brushes is not easy. Nylon is about the only suitable material but it is difficult to form into complex shapes, nor can it easily be glued: specialist licences are required for the solvents involved.

When the armature gets old, it starts to throw its varnish and seize up. It’s not enough just to clean it out, as problems will continue and it could let you down at any time: the only proper solution is to have it rewound.

Tony’s advice is to get a good instruction book and follow what it says to the letter; then, if something goes wrong, describe the symptoms to a specialist, who may be able to diagnose the problem remotely. For example, a car that has been standing, especially over winter, may well not start for the simple reason that the points have oxidised and need cleaning.

Intermittent misfires are usually the most difficult to trace but the majority are due to condenser weakness, short circuits (especially partial shorts inside the coils), poor connections inside the magneto, incorrect points material or even too wide a spark plug gap. Look after your HT leads – failure of just one lead will overload the magneto and could burn out the whole unit.

Original magneto condensers made of mica last 100 years if undamaged but are not available new, though the modern X2 type of radio capacitor of 0.22 or 0.3mfd is proving serviceable. Lucas wax paper condensers can be found NOS, but are often useless. Using the correct material for points is vital and here it is important to keep with the original manufacturer’s choice. Some are platinum/iridium, some are tungsten. The former cost at least £70 a set, the latter can be cobbled together from ordinary distributor points.

Age and high mileage do affect magnetos, as Tony Stairs explains: ‘Physicists tell me it’s not possible, but it’s true: with age, the molecules in the iron get tired. They’re swapping magnetism from north pole to south pole at least twice a revolution and, the more times an armature turns in its life, the more sluggish the molecules become, especially over 2500rpm. There is not much you can do – there are a few tricks of the trade to get around it, but I can’t reveal those!’

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