EL016 ELECTRICAL

I would appreciate your advice on the following disturbing mechanical break down which I went through some 6 months ago. The bike is a Thruxton 1967 with manual advance/retard Lucas magneto. Deciding to enjoy a nice sunset I started the bike. No problem. It started with the usual amont of right leg power and subttle combination of cylinder head ventilation, carburator flooding and throttle. Not to mention some fine tuning on the ignition advance position. Went very well as usual and then stalled after some 15 minutes without obvious reason. No way I could start it again. Even down hill. Plug and sparks on the plug were fine, petrol was in the tank, carburetor looked ok, points were as they should be, etc.... I came quickly to the suspicion that the timing has splipped. And finaly had to accept that I would have to go back home on foot. Fortunatelly just few miles away. Checking the points rotating base plate made me suspect that the driving gear on the mageto shaft was loose as I could feel some movement. The removal of the timing cover enabled me to realize that something far worse had occured, and a complete dismantling of the magneto confirmed the following. The spline connection between the driving shaft and the brass rotating cage had sheared. The isolation compound in which the coil on the rotating cage is embedded seemed to have somewhat melted as the inside of the magneto body was covered with it. The lot was sent for repair in the UK, the magneto is now back in operation and everything is back to normal at least for the time being. What happened ? A comment first. The magneto is supposed to be a Lucas Competition one. So does it say on the nameplate. But the rotation cage is a brass type as a competition one should be of steel. So some fiddeling may have occurred here in the past. Am I correct here? I could not see whether the spline is complemented by brasing to secure the mechanical connection between the shaft and the cage. As I did not want to dismantle the unit after its repair I don't know what is the current configuration. What sort of electrical problem could have resulted in the isolation compound to melt to the extend of covering the inside of the magneto body. Or is it normal that after a while some of this stuff should be centrifugated out? Never seen this before. Could this deposit have resulted in some excessive mechanical braking on the cage while rotating resulting in the brass side of the spline to shear? Or is it just a mechanical failure? I am anyway surprised to see a spline connection into a brass body which is a soft material. Whilst everything is back to normal I am puzzled by this unpleasant trick from Lucas. - Gilles Glachant 10/12/2002

One of two things happened (apart perhaps from your magneto not being an original Lucas competition mag.).
The most likely cause of the problem was that the 'shellac' on the armature windings melted, was thrown out, filled the small gap between the armature and the mageneto body, caused it to seize, and so snapped the drive shaft.
This is absolutely normal!
It takes from 20 to 50 years for the shellac (a natural organic compound......do you know what it is made from!) to go soft. It depends on heat/humidity/usage/revs/etc. But it always happens in the end. I do not criticise Lucas too much (nor BTH, since it also happens for them), because shellac was still the best insulator available to them at the price that a motorcyclist could afford.
Your armature repairer will have now used a modern synthetic insulator, but who knows how that will behave in 40 years time?
Also I do not criticise the mechanical design of the magneto drive - it works fine until the armature seizes!
(I did not know that the competition armatures used steel rather than brass, but it makes sense)
The other reason that the drive shaft can snap is if you use too thick a gasket between the magneto and the crankcase. On some magnetos this can cause the gearwheel to rub on the crankcase face, since there is sometimes only a few thou clearance with the correct Velocette gasket, which is a very thin one. This puts the shaft under heavy stress. So it snaps! One of mine did half way through an important race. But my own fault for fitting a non-standard gasket. - TW 10/12/2002