The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: Alan @Ncl on 21.01. 2011 10:17

Title: K2F Magneto HT Slipring Grooving
Post by: Alan @Ncl on 21.01. 2011 10:17
Hello to all again.  As a relative newcomer, here is my second thread to this amazing forum, coming out of my efforts to resuscitate my old A10.

Does anyone know how a magneto HT slipring can become as badly grooved as the one shown in the attached pic?  The mica is worn in a groove, a bit wider than an equivalent brush width but not the whole track width.  It is 2 to 3 mm below the brass and extends round the full circumference of the mica.  Amazingly, it was still working when I last ran the bike (15 years ago) although not surprisingly, was prone to misfiring (but only at higher revs as I recall) and breaking brushes. 

As part of my current renovation project I managed to get hold of a relatively cheap second-hand magneto that was missing several parts and came without any promises.  Happily, after a bit of investigation and fitting some missing bits from my old mag, it seems to be sparking nicely when turned by hand.  Have not run it yet and will fit different brushes and holders but would obviously like to know what caused the problems in the original mag, before proceeding further.
Title: Re: K2F Magneto HT Slipring Grooving
Post by: Brian on 21.01. 2011 10:43
This can be caused by a couple of things Alan.

If the slipring has been replaced by a poor quality reproduction this will happen. Another cause is the brushes, some after market ones are too hard. Another is how they were fitted, its important that the springs they attach to are pushed fully down into the brush holder. When they are fitted you should be able to push the brushes down flush with the brush holder with very little resistance.

I have had the exact same thing happen to me due to a poor quality slip ring and like you the bike was still running fine.

I would strongly suggest that if at all possible send your maggy to a reputable specialist and get it reconditioned. If you are confident you can get the armature rewound and do the rest yourself. With our A10's (and most old bikes) the maggy is the heart of the bike and has to be in good working condition if you want a reliable bike.
Title: Re: K2F Magneto HT Slipring Grooving
Post by: jjbsa on 21.01. 2011 11:26

I had this problem first of all many years ago on an A7.  It had pretty likely been caused by the brush not being fitted into its holder so that the side of the spring acted as a gentle cutter on the slip-ring.  The groove worn was quite deep, like the one in your photograph.  My father kindly fixed it by roughing up the indented area with sandpaper to act as a key for the Araldite he was going to apply, cleaning off the roughened area very well with something like meths or lighter fuel, and then applying the Araldite.    (This was in the early 60s when Araldite was still something of a novelty).  After it had set, he then turned down the built up area on a small lathe till the slip ring looked like new.  This magneto gave faultless service for years after.

From what I've learnt since, I'd add the following comments.  When I use  Araldite to build things up I prefer to use the slow setting variety and cure it in an oven at about 70° C.  When treated  this way the Araldite sets into a clear glass with no bubbles  in it, so that when you machine it you are unlikely to create pockets where you have cut into voids in the glue (if this happens you need to have a second go with the Araldite).  To stop  the Araldite running or sagging, I leave it at room temperature for about half an hour before I put it in the oven, when it starts to go a bit like a jelly,  then when you put it in the oven it doesn't run.    Also, in the oven the Araldite queues a lot more quickly, a couple of hours is plenty.  When you're machining this sort of stuff in a lathe I like to use a really sharp high-speed steel tool  with plenty of top rake.

I agree totally with Brian's comment about aftermarket brushes.  There seems to be a lot of variation in hardness.  I always have a look around to see if I can find a useable brush that might once have come from Joe Lucas.  I remember one of the electrical specialists telling me that soft brushes could also cause problems because they can create dust which is conducting.  I have a dim recollection that originally the three types of brush in the magneto might not have been all of the same hardness.  Can anyone throw any light on that?
Title: Re: K2F Magneto HT Slipring Grooving
Post by: Alan @Ncl on 21.01. 2011 13:54
Thanks jjbsa and Brian.  You have provided some very useful comments here.  I will certinly keep an eye on the brush fitting and look out for signs of premature wear if and when I get running again. I think the slipring is original but the brushes were not and I suspect the holders may not have been either.

I too remember the 'invention' of Araldite (I first saw it in about 1962 when I was an apprentice).  It seemed like a miracle back then.  Sadly my slipring is beyond repair though, and the armature is shot anyway (open circuit reading on HT) though still capable of the occasional feeble spark (must be jumping the break in the winding).  Its on the shelf now as a spare and may get rebuilt sometime (funds permitting). I guess it depends how my replacement mag performs.  Resistance readings are certainly good (about 0.6 ohms on primary and 4.2k ohms on secondary).  Slip ring on this one has a bit chipped off the side flange though where it looks like somebody has tried to pull the armature without backing off the safety screws (part reason why it was cheap, I guess).  I am hoping it does not matter too much.
Title: Re: K2F Magneto HT Slipring Grooving
Post by: JohnH on 21.01. 2011 23:25
Araldite - brilliant stuff - never be without it. I used to work for Evode and one of my mates worked in the development lab working on epoxy resins and hardeners. We went on a trip to Scotland - me on my 197 Fanny B and him on his Velocette MSS ..... yes, yes, I know ... anyway, around Penrith, my carb banjo started leaking two stroke mix all over the place .... the thread was screwed - there's a pun then. Without further ado Jim went and bought some Araldite (very new then - about 1961) and put a big gobbet all round the banjo. "What about the petrol?" says I. "No problem" says Jim - "it isn't affected by petrol." And sure enough, the next morning it was fine - no leaks ... and it stayed that way for the rest of that year until I could find a new carby. Brilliant stuff - never been without it since. JJBSA is absolutely spot on - the best stuff is the non-Rapid type and it will cure better in an oven. Sorry if this is off topic - couldn't resist it.

Title: Re: K2F Magneto HT Slipring Grooving
Post by: terryk on 22.01. 2011 14:00
Good info John H. I have used it many times on wood etc but never thought about using it like that. I will have to remember to buy some for the toolbox you never know what you might need it for.
Title: Re: K2F Magneto HT Slipring Grooving
Post by: JohnH on 22.01. 2011 15:46
Given the space 2 tubes and a plastic spatula take up, it's a must have in my toolbox - you won't regret having it on board!
Title: Re: K2F Magneto HT Slipring Grooving
Post by: A10Boy on 08.02. 2011 16:47

Back in the early 70's as lads me and my mate got hold of a german Victoria three speed moped. [How much would that be worth now] to ride around the fields. The carb had a screw on float bowl that we took off a lot to get crap out of the carb. The bowl had nut shape cast into the bottom so you could get a spanner on it. That was soon rounded off and we fixed it with araldite. We dolopped a gob of araldite over the rounded nut and when it had set, we filed it back to the nut shape. It was still there months later when the bugger caught fire and destroyed itself while I was riding it.. Happy days..
Title: Re: K2F Magneto HT Slipring Grooving
Post by: JohnH on 09.02. 2011 19:06
Now that sounds like the Bantam that myself and about 10 mates rode to destruction when we were all about 12 (that would be about 1956 or so). Unfortunately, there was no Araldite available then and I've always felt really guilty  *eek* about the way we treated that little Bantam .... great fun though.  *smile*