Hello all

I see there was a lengthy discussion on magneto oil seals, but as my issue is slightly different, I thought I’d start a new one.
The bike coming back together nicely after major strip and I noted (albeit not that carefully) that the magneto oil seal had failed. I ordered what appears to be the only standard part (65-2316). When it came, I was surprised to see it was too big (see pic). After a slight scratch of the head, I noted the case had been repaired with an insert that takes a smaller seal.

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I read codes off the failed one and got a replacement easily enough to replace the old one. I thought all was good. However, when I put it altogether the pinion sits a bit high in the orifice as in the pc. I guess this explains why the old one failed on 1 side!

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I see comments about here on shimming but that would raise it further, I need to lower it. Has anyone seen this before where the mag sits high on its pedestal? Looking at base of mag, it seems to sit on those 4 steel pegs that locate in the 4 channels in the crank case.

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Do you think I need to grind them down a few mm or do I need to skim the base of the mag?

Any advice greatly received as ever.

Cheers
Neil

email (option): nlhclarke@gmail.com

It looks to me like the points end of your mag has eaten its way into the mag platform. The only way to cure it without major surgery, would be to install the pinion and shim under the back end of the mag to get the best possible mesh of the teeth. Maybe sacrifice a set of feeler gauges to experiment with. There must be a little bit of lash in the teeth. Too tight and it will wine. Ron

email (option): ronpier@talk21.com

The mag destroying the aluminium base is by now after many decades a common fault on the B and M series. However your's looks brand new and nothing like my M21 one, that incidentally was utterly shagged.

As Ron has said you get the gear teeth to line up perfectly and then deal with wherever the seal is supposed to be.
As new some engines came with packing shims between the mag base and the platform, shimming the left pegs might work.

Ian will know, but the replacement seal 652316 can't have been the wartime part, which in all probability was felt [as was on my '54 Bullet]. If you could locate some felt sheet you could using punches make an off centre hole in the round felt seal, should someone previously have fouled up the timing case location.

email (option): jeremy@clogmaker.co.uk

Hello All again

I see various comments about magneto seal wear/ misalignment as a result of loose strap and subsequent wear of the crankcase platform and /or the base of the magneto. I have not seen these surfaces before and already appear to have various shapes cast into the faces; particularly the Maltese cross cavity on mag body and similar shape on crankcase.

Can anyone tell me if these faces are excessively worn or is it minor?. I can see the small indentations along the slots in the crankcase where the steel pins have pushed against the side walls but is this bad?

Also on the mag base there is a ridge of Aluminium along side 3 of the pins that I guess would sit in the crankcase channels. Is this normal or has all around it worn to leave these raised ridges ?






Hopefully these pics load

cheers
Neil

email (option): nlhclarke@gmail.com

They look awful to me Neil.
For the mag I would remove the dowels and face it flat.
The crankcases I would weld and re-machine.
Then make a shim to take up what was machined from the mag.
I'd also replace the mag gear and the one it runs with.

Mark

email (option): pes.sales@btconnect.com

They look excellent to me, wish mine was that good; but then I'm not an engineer, just a rider and back street mechanic at best. I'd get a professional opinion on the state of your gear teeth before you spend money. From an engineering point of view I couldn't argue with Mark but again I wouldn't follow his advice either. I can't afford to.

email (option): jeremy@clogmaker.co.uk

I posted a reply after your initial query about the state of your mag and platform. Maybe you missed it? You can see exactly why the mag has tipped back so far and worn your seal away. Ron

email (option): ronpier@talk21.com

Those are not casting marks, they are caused by extreme wear of the mag platform. Apart from the four machined slots that accept the pins in the mag body the platform should be COMPLETELY FLAT...
The mag platform either has to be remachined or welded and remachined as Mark described...
Depending on the depth of the wear you can machine the mag platform flat, removing the thickness of one material gauge (18 gauge or 16 gauge)...This avoids the potential hassle of welding the cases and the correct dimensions can be restored my making a plate to fit under the mag of the appropriate gauge material....If however, there is too much wear and you simply can't remove enough material to make the platform flat again without it being too thin, then welding and machining is the only option other than replacing the crankcases with a better set!...When setting up the cases for machining the edge of the mag platform at the points end is never worn as the mag does not cover that part of the platform..It's probably approx. the last half inch...This can be used to clock the platform level before machining...
Likewise with the mag..Only a limited amount of material can be removed from the mag base before breaking into the fibre drive gear housing, so examine it carefully before commencing work...
To avoid all these problems after rectification it's only a matter of tightening the mag straps and keeping them tight to prevent the mag moving around...
So, make that a weekly check when running the bike or better still fit a locknut onto the magneto strap bolt and tighten it against the mag strap trunnion at the opposite end to the head of the bolt....Take it from me and others though, that mag platform is bad and should be properly repaired if you are ever going to restore any degree of accuracy and correct function to the set up...Ian

email (option): ian@wright52.plus.com

There's problem with the straps that are now sold. The strap metal is thicker and thus doesn't immediately bed in as well as the originals. One has to keep a check on the straps, as over time they more correctly follow the line of the magdyno and thus slacken; quite apart from any locknut, or not.
As to Ian's last resort of having to get another set of cases: well it's undoubtedly the correct option from an engineering view point; but I chose what Ron has suggested and it's lasted for 8 trouble free years without noticeable degradation. My M21 is worth perhaps £2000 in today's market. Fixing all it's faults would cost far more than it's value and add not one jot to my enjoyment of it.

email (option): jeremy@clogmaker.co.uk

Everyone to their own but I wouldn't be inclined to promote some of the procedures described as good solutions...
I ran a business restoring heavyweight BSA single engines for nearly 20 years after I left my engineering career and have owned various examples for over 50 years...I try to post what I feel is the best procedure based on my experience but what people do with that information is entirely up to them...
Personally, as an engineer, I feel many of these 'fixes' should be avoided if at all possible and should only be employed as a temporary fix until the job can be carried out to a decent standard, not as a prefered way of working...Are we really going to make a virtue of deliberately avoiding doing things properly?....I'm guessing you don't make clogs with such a gung ho attitude....Ian

email (option): ian@wright52.plus.com

You win the prize for the most magdyno platform wear for 2025! What you have is very bad. As Ian stated the magdyno and platform should both be perfectly flat. This is a very common problem and I expect to see it anytime I work on a B or M series. Check the mounting strap anytime you ride the bike. They do come loose.

I fix the problem by milling the platform flat and add a aluminum plate to bring the height back up. If really bad, I sometimes have to weld it.

When the magdyno is this bad, I usually look for a better magdyno body instead of fixing the old one.

You are going to have to visit a machine shop and possibly a welder to fix this problem.


If you have not taken the magdyno apart, I would do it. When the magdyno sinks down in to the platform it can take away the gear clearance and put a side load on the bearings and destroy the insulating cups.