Magneto problem

Started by paul.w, August 03, 2022, 08:33:26 AM

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Rick Parkington

Well done Paul! That's exactly what my 9A armature looked like! As I say I am sure the massive sprocket overhang is the cause so make sure the chain is never too tight!
Yes, the bolts are a faff. Not sure what you've got but as standard they seem to have used two studs with nuts and the other two are just locating pegs. From memory I think on mine I fitted an allen bolt in one hole and a reduced hex nut on the second stud to make it easier to adjust the chain. I'm a great believer in setting things up so you can do them at the side of the road if needs be.
Cheers R


well, that explains a lot.........

Mag man has fitted a new armature.  All back together, and started first kick  :). Inserting the bolts securing the mag to the platform was a nightmare though, it needs a small child with prehensile fingers 6 inches long.


A really silly thought(!): Was the mag sprocket properly tight on the shaft? It has been known for the sprocket to loosen if the taper in that and the shaft are not in good condition. Having said that, since you have taken the mag off, you would have noticed!


Rick Parkington

Ha ha! Fair enough Paul, Velosolex it is then!
Cheers Rick


thanks for that.  I read it and decided I'm consult a magneto repair man so I've just dropped it off!!
He reckons its not common but he's seen it before and they do sometimes break at the other end.  Worth taking to an expert, he reckons if I'm lucky he might be able to silver solder a repair.  The downside is he's busy so about 6 weeks.
I could have got it apart following your comprehensive instructions, but I'd then still have to repair it and it's better that someone who has the experience do it rather than me making a mess of it and making things worse.
On the upside, at least it means I would be bothering the forum with more questions for a couple of months!

Rick Parkington

Hi Paul,
Curiouser and curiouser. It is very hard to see why the other end would break but here goes!
First job is to remove the fibre wheel, to do this you need to lock it and rather than wedging something in the fragile teeth, it's worth making the official tool - a bit of 1/4" rod bent 90 degrees at either end so one end hooks into the hole in the clutch plate thing under the nut and the other into the hole in the body casting where the dynamo stud/sleeve nut goes. Keep the bent ends as short as possible for strength,  the nut is usually pretty tight and the longer the ends are the more leverage to  encourage them to bend and slip out of the holes.
Nut off, remove the clutch plate, star spring, hardened washer and fibre wheel and the two keys in the shaft.
Next, if you haven't already, remove the complete HT lead pickup, held by two screws to the casing and also remove the earth brush holder - a flat headed brass screw in the side of the body at the points end, it should contain a spring and carbon brush. There shouldn't be a 'safety gap' screw on am MO1 mag - a pointed screw for the spark to jump to if there's a problem, like a failed plug - but just make sure there are no other screw heads visible around the equator, so to speak, in line with the HT pickup and if so take them out.
Now undo the four points housing fastenings - some screws, some nuts and the points housing should come off, maybe needing a tap with a screwdriver handle. There may be thin brass or aluminium shims, like gaskets, beneath so be careful not to damage them. With the cover removed the ball pack should remain on the armature and with again a light tap on the drive end shaft to free the bearing, the armature should slide out easily.
Then, I guess, all will be revealed!
cheers R 


Hi Paul,

from my point of view it depends on your skills. Yes, it's a nuisance if the bike needs to stay in the shed. However, if you give it to an expert (who is even around the corner) it might be a quick inspection and repair without the risk to do it wrong. That is what I would do with my limited magneto knowledge. Just my two pennies.

Cheers, Thomas
1946 BSA C11
1937 Sunbeam Model 9
... and a scratched Hyundai
(MSCR member)


I've taken the sprocket end plate off and the gear stayed in place, so if there is a break I think it might be at the other end near the points.  How easy is it to take off the "points" end plate?  I'm tempted to at least have a look before I go looking for a magneto repair man (we do have one fairly locally, but he never answers the phone - one has to visit and hope he's there!).

Either way its probably going to be months before its repaired - assuming it can be (I wonder if replacing the shaft means a total rebuild and rewind).  Its a long shot, but does anyone out there have a spare they might be prepared to part with, otherwise it looks like the bike won't be on the road again until next year.  I might be reduced to riding around on a Velosolex!!

Rick Parkington

Oh dear, sounds like a broken shaft after all. If you remove the cover on the drive end I guess the end of the shaft will come out with the fibre wheel attached. The actual armature itself is built up from three bits but held together with screws and dowel pins, so it's unlikely to come apart.
There does seem to be a weakness here,  my '32 Sunbeam is the only other bike I have ever seen with a broken shaft and I wonder if it has to do with the considerable overhang on the Sunbeam mag sprocket, it sticks out over an inch beyond the threaded end of the shaft and if the mag chain ends up too tight it will exert huge leverage on the largely unsupported shaft.
Interestingly Sunbeam made a little outrigger bearing housing for the M90. It  fits on the back of the mag chain case so that the sprocket runs in a bearing - implying they were aware there was a problem too. I fitted one of these housings on my 1928 Model 9 recently and I'm glad I did now!
At least being an MO1 it will be easier to replace than an MS1 type - but it's not what you want during good weather!
Cheers Rick


three photos - no sign of any breakages




photo 1


Thanks for that Rick,

Your description of the M01 is spot on as what I have.  There are no obviously broken parts, I've removed the contact and lifted the points and can see the "c" shaped cutout on the base of the point unit and corresponding one on the armature end.  There is no sign of damage anywhere but I can rotate the armature end and the input shaft remains still.  There has to be a break actually somewhere on the armature shaft itself I suspect, which would be bad news.
I'll try to upload a couple of photos - if/when I work out how.

Rick Parkington

Hmm, you do seem to be under the influence of gremlins at present Paul!
My 1932 9A came with its original MS1 mag/dyno and I was surprised to find the actual drive shaft had broken off the armature, but although sill held in place by being attached to the large fibre dynamo drive wheel, it wobbled about and if that was your problem I think you'd have noticed when you took it off the bike. Also the MS1 has a ring cam with the usual usual taper and key points set up you're familiar with, so I suspect your mag is not one of those but maybe the later M01 type with a face cam, where the moving point is like a diving board and opens out toward you. If so, the points unit is cast mazak with a sort of C shape cutout that fits over a similar shape on the armature end, secured by a central screw. When people tighten the mag sprocket they inevitably take hold of the points to prevent turning and put a lot of stress onto this delicate part and if the mazak has broken/crumbled, the points will be free to spin round on the armature shaft as you describe - I  guess you'd have noticed broken bits so maybe it was already crumbled and someone hoped just the screw would hold it.
If you remove the 'diving board' moving contact, held by a screw at the pivot end, you will see the central screw that holds the unit and can get it off. Much more likely that this is broken than the end of the armature, so fingers crossed there and luckily being a part made into the 1960s they're not too hard to get - although the thread for the moving point is often stripped. IT is possible the until has just come loose but I'm not sure the retaining screw can come undone enough to disengage without hitting the moving point above. 
Anyway hope this helps.
Cheers Rick


I think I need o book a relationship Councillor for my Model 9!!
Latest problem....  I was riding it a few days ago and it coughed a couple of times and stopped.  Identified lack of spark and removed Magneto.  Discovered I can turn the sprocket and points are no longer rotating.  Alternatively hold the shaft and turn the points - so there is a disconnect somewhere in the magneto between input drive and points.

The timing gears are OK, and I've removed the points to see that they are effectively pegged to the end of the shaft so the problem is inside the mag.  I was hoping/expecting to find the points were on a keyed taper (common in other mags I've had apart) but apparently not.  - so the problem is inside the mag.

Before I either take it to a Mag restorer or take the end off to look inside can someone tell me what I'm likely to find.  As I rotate the points while holding the shaft I can feel a bit of varying resistance - which is what I'd sort of expect if there was a knackered woodruff key in there.  Before it broke it had a decent spark so I don't think there is a fault with windings

Thanks in advance for any advice