The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => Lucas, Ignition, Charging, Electrical => Topic started by: a10gf on 08.12. 2010 11:29

Title: magneto: fibre vs aloy drive gear
Post by: a10gf on 08.12. 2010 11:29
Reading this post (,3533.msg24199), it reminds me I've always wondered why fibre drive was used originally on all (?) mags. Without thinking too deeply about it, I have the impression it has an isolation purpose, as the armature should ideally be isolated from the mag body (& frame) > therefore the use of isolation washers between bearings and body. An alu sprocket would imply electrical contact to the rest of engine. Opinions, enlightenment (or proof of my utter ignorance) welcome.

Title: Re: magneto: fibre vs aloy drive gear
Post by: groily on 08.12. 2010 12:02
Not on all mags by a country mile a10gf!
Chain driven ones on lots of singles as well Norton and Enfield twins, all gear-driven AMC twins, etc used steel bits. I'm using an SRM alloy one on my A. Vincent used fibre with ATD on twins until they went to coil ignition (which would have been very sensible had the electrics been a bit more robust), and of course BSA did. Triumph I don't know but I think they may have used fibre on some ATD jobs like on early Speed Twins.
The mag armature is earthed through the earth brush anyway. The insulators behind the bearing outer races are there to prevent any tracking across the bearings which might start to eat away at them. And the sparking plug has to be part of a route back to make the circuit . . . otherwise, no sparks.
I think, but don't know, that the idea was that the fibre pinion was a) quieter (true) and b) was a point of failure in case of seizure elsewhere. The teeth would strip - ha! don't they! - if the mag seized up. Nothing more expensive would break.
However, the only Vincent I know had a mag seizure due to molten shellac glueing it solid after stopping it on a very hot day, and not only the fibre gear stripped as well when coming back to try start it after a beer stop, but the kickstart quadrant broke as well. Some glue!
Bad hair day if ever there was, and about 650GBP to repair all the damage.
Title: Re: magneto: fibre vs aloy drive gear
Post by: beezalex on 09.12. 2010 18:15
From what I understand, it was to force current to go through the earthing brush, not the bearings.  Ball bearings don't like it when the balls arc into their races.
Title: Re: magneto: fibre vs aloy drive gear
Post by: Stu55Flash on 09.12. 2010 20:29
There maybe a non technical reason like the factory had loads of fibre sheets left over at the end of the woer and used these up instead of buying more aluminum??

I once worked in aircraft avionics repair and we searched for reasons as why a particular circuit had 2 resisters in parallel instead of one.  It turned out the manufacture had loads of double the necessary value resistors to hand and used these instead of buying more.