Magneto Earthing Brush and Contact Surface

[RichardL]

Starting the rebuild of the magneto for my A7 and looking for comments and advice on the dimple in the earthing brush comtact surface. All free-form thoughts, advice and humor are welcome.  To me, the dimple on the rotating contact surface (about 5-6 thou deep) seems like it might coincide with where the spark fires due to the slip-ring contact.

Looking forward to ideas for fixing, or, for that matter, ignoring it, if anyone has done so with successfull results.

Richard L.

[berger]

well I don't know a lot about these things, except what I've just found out earlier with copious amounts of oil in mine, but before I put the comp maggy back on the bike I saw the similar or maybe  bigger dimple again that has been there for umpteen miles------  actually a great big crater in the brass on the slip ring and it hasn't bothered the bikes performance or starting so I forget about it.

[Triton Thrasher]

People turn the brass down in a lathe, to make it level with the bottom of the dimple.

[trevinoz]

What TT said.

[muskrat]

G'day Fellas.
But why is it there?
Cheers

[groily]

It's the place the return current lands up when the mag fires. So there'll be 2 dimples on a twin often.
Turning true is the answer. But don't go too hard at it, don't want to break through the brass and hit the steel dowel pins lurking under the surface. In very severe cases the dimple is deeper than the brass depth available to machine, so the best thing to do there is skim to the limit and then smooth out the 'on and off ramps' of the dimples with emery or whatever to make the best of it.
The dimples come back though - they're like acne.
A secondary earth brush on the rear of the cb backplate can maybe reduce the dimpling where one's fitted or could be fitted. On many mas there is ONLY a cb mounted brush and the armatures remain unscathed.

[RichardL]

Thanks guys. I was pretty sure the prescription would be turning, but thought I'd ask about the alternatives. Turning would be a breeze if the membership DIY machine shop I used when I did the timing-side-bush had not gone belly-up due to the pandemic. I always thought they were on thin ice. They went under pretty early-on.

The ramps on my dimple (calm down) are somewhat smooth already, but maybe just right for the brush to use the ramp for a motorcross jump. Groily (or anyone, for that matter),  do you know the approximate total thickness of the brass cylinder?

Richard L

[RichardL]

... But don't go too hard at it, don't want to break through the brass and hit the steel dowel pins lurking under the surface.   

Uuuuh, I guess wall thickness is not the issue. Do you know the approximate depth of the dowell pins?

Richard L.

[RichardL]

Could that spot be built up in brazing without frying the coil?

Richard L.

[JulianS]

Cannot give the depth but the photos show my armature drive end when replacing the capacitor.

It is a stainless part but the dowels are the same place on the brass ones.

I think the second photo shows it had been trued up.

[RichardL]

Juslian,

Thanks, so much. Very enlightening to know the sit inside the wall thickness.

Richard L.

[Swarfcut]

    Back in the mists of time, when I thought I had the last remaining  armature in the world, we stripped the drive end from the armature, removed the condenser and made a good repair with silver solder.  Bronze welding was another option but we went for the least amount of trauma to the part. A light skim and a polish made it good as new. Removing material from the running surface will extend the carbon brush from its holder, whether this is of any consequence until the brush shortens enough to pop out is one of those things. The brush under a brass type points carrier, as suggested, sounds a much easier fix.

 Bit different now, there seem to be more magnetos and parts available in various states of disrepair or running order than ever before.

 Swarfy.

[trevinoz]

That would be a special racing armature, Julian?
I've got an N1TT magneto with a stainless drive end.

[JulianS]

Yes it is from a K2FR.

[Greybeard]

The ramps on my dimple (calm down)