I have a couple of questions about the MO series magneto's fitted to M20's.
What's the known acceptable run-out of the brass slip ring surface, and also the earthing brush surface.
Is 2 or 3 thou too much considering it would cause brush movement in its holder. I've shimmed my mag with no noticable end float.
On several spare rotors I've checked there seems to be a step worn into the earth brush ring at a point about 10 degrees or so before the points open. When the mag is turn over by hand, there is a lump, or pull from the magnets felt, and they seem to develop this step at that point. Maybe from excessive play in the bearings, in the past.
I'm trying to skim the brass surfaces true in the lathe, relative to the bearings, but it is difficult to set up.
Also what is the thread of the earth brush. It is very close to 3/8" BSCY, but I want to make sure.
Thanks very much.
Keith

email (option): keithatkinson@hotmail.com

Hi Keith. Greetings from Cairns. Never seen a figure for radial runout on the armature or HT slip ring; I doubt that a couple of thou is an issue. A quick touch in the lathe should reduce it to zero easily enough. I pull the bearing inners off & hold the armature in the lathe chuck at that point with a centre at the other end. Usually O.K.
When you have the armature rotated to magnetic neutral within the magneto body, i.e. when you can feel it 'latch' into that spot that it wants to hold, that is the point of maximum static magnetism. The better the overall magnetism, the more obvious the effect. The points will be opening around 5 to 10 degrees later, so maximum current is induced in the coil at this point. A little arcing at the earth brush would give the effect you mention. It's normal. Polish it out if there is a bit of a step.
I measured a mag. body & brush holder thread; it certainly does appear to be 3/8" cycle thread. Too big for a B.A. size. Not much else it could be really?
Hope that helps. By the way, I have a magnetiser here at work.

Thanks for the good advise. Unfortunately, It is difficult to machine it as you suggest. My 3 jaw self centering chuck is out by 4 thou. Also the live centre hole at the drive end is off centre. The step in the earth ring is actually about 10 thou deep, as wide as the brush, and about 1/2" long. The magnetic pull on the rotor must take up the play that developed in the bearing before, at that point, causing the distinctive step. It doesn't look like arching. Ive found a couple of deep groove ball type bearings that fit the shafts, so eventually I'll be able to derive a method of skimming the ring surfaces close to zero. The magnets in this thing, are suprisingly strong, so they shouldn't need re-magnetizing.
Never thought I would spend more than 20 hours, trying to restore a magneto.
Cheers Keith

email (option): keithatkinson@hotmail.com

hi keith,if you grip a piece of bar with a bit of flat bar welded to it at 90 degrees in your chuck,make yourself a carrier
ie a boss that will slip over the shaft,with a screw to grip the
armature.
next turn a centre on your bit of bar with your topslide over at 30 degrees,everything should now run nice and true,is this making sense?
for turning brass ideally your tool should be flat at the front
(not raked as with steel, ali, copper )
any questions please ask,i would do you a sketch but ive no scanner
hope this helps cheers rick

email (option): richardholt@rocketmail.com