Hints and Tips - Magneto Issues

From: Graham Orme-Bannister
Sent: 23 June 2000
To: Adrian Roger Twelvetrees

....... Back to matters pre-war. The magneto on my Riley packed up recently after only a few hundred miles, but quite a long elapsed time, since the engine rebuild. Fortunately I had a spare and while I was at it I changed the plugs and HT leads. When I took the old plugs out the gaps were some 30 thou., I set the new plugs at the recommended 18 thou. and the previously slow cold starting miraculously became instantaneous. Now to the point; reading some recently acquired copies of early Riley Register Bulletins I came across a reprint of a 1930's Riley Technical Bulletin to the effect that a common cause of magneto failure was excessive plug gaps putting too high a resistance load on the magneto windings leading to overheating and failure. It fits my situation exactly but I had never heard this before. Do the electrical experts in our number, of which I am not one, accept this thesis.

Graham Orme-Bannister
Alresford, Hampshire, UK

From: Adrian Roger Twelvetrees
Sent: 23 June 2000
To: Graham Orme-Bannister



On the topic of magneto failure I am extremely sceptical about a largish plug gap damaging the insulation. There should be a safety spark gap somewhere on the high voltage side, usually under the rotor arm, which sparks before the voltage rises to excessive levels. The energy dissipated in the magneto might be expected to be less if the plug gap is larger, since the current will flow for a shorter time. In any case, the energy involved must be tiny compared to the conducted heat from the engine. Older magnetos fail from ageing of the enamel on the wire, or breakdown of the insulating tape between the layers of wire. This is a well known "life" effect of older insulating materials, which can be delayed by keeping the winding cool and perfectly dry. There should be no such failure mode in modern materials, and the problem is almost certain to be a breakdown of the capacitor. So called infant mortality is a problem with electronics. This is overcome on high quality electronic equipment by "stress testing" at high temperature to weed out incipient failures before shipping to the customer. If the winding has broken down, then the reason is likely to be a quality failure in the layering and interleaving of the high voltage winding. Independent ignition supplies will supply you with a capacitor by mail order for a few quid if you feel confident enough to fit it yourself.

Best wishes,


From: Peter Jacobs
Sent: 23 June 2000
To: Graham Orme-Bannister

Graham, Regarding your magneto, if the plug gap is too wide then the voltage generated in the magneto has to be greater to the make the spark jump the gap. The danger is that it may find an easier path by jumping between layers of the fine wire windings in the magneto's coil. Once this has happened the insulation may be broken so that it always chooses this path even when the plug gaps have been narrowed. This means a rewind is necessary. The coil's insulation can be degraded by prolonged exposure to damp. The best place to keep your mag's when the car is off the road is in the airing cupboard!

It is usual for magnetos to contain a "safety spark gap". This is designed to permit the spark to jump to earth in circumstances such as a plug lead becoming disconnected, and is set so that the magneto's coil is not overstressed. Unfortunately, these safety gaps are not universally understood, and people widen them in conjunction with wider plug gaps to get a bigger spark at the plug. It may work for a while, but in due course the coil insulation will give way.

Therefore, always maintain plug gaps at no more than 18 thou when a magneto is in use, and regularly check that there is minimal electrical resistance between the each plug terminal and its segment in the distributor cap. Do not use anything but copper cored HT leads, and avoid plug caps which incorporate interference suppressors.

Hope this helps.

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