Magneto rotor weirdness; rotor rubbing against the starter idler.

[h0dgep0dge]

I'm elbow deep in a motor rebuild, and I've run across something (I thought) rather strange. You can find some pictures in this (https://imgur.com/gallery/nCyKDm3) imgur album, which show one of the 4 burnished spots on the rotor. At least when I have the magneto cover removed (and the starter idler is only being supported on one end), the gap between the gear and the rotor goes to zero and they grind against each-other.
I entertained the thought that the high spots are machined to give a small amount of clearance, and slop introduced to the idler shaft by removing the support on one end caused it to start rubbing, but the spots look burnished, so I think it's been rubbing while the motor was running.
If I was confident that it was only a manufacturing defect I would just file down the high spots. However, I am not that confident. Is anyone aware of what might cause this, that should be looked into further?

[Dadpole]

To me, that looks like a gouge mark caused by a stray item. The starter clutch (behind the rotor) has three (or four) metal rollers held in by a spring arrangement. It is possible one has escaped and has caused the markings.

This is based only on what I can see in the photos, but were there any metal fragments anywhere?

[h0dgep0dge]

The starter clutch (behind the rotor) has three (or four) metal rollers held in by a spring arrangement. It is possible one has escaped and has caused the markings.

Thanks for the reply. I anticipate I'll be taking off the rotor, if only to deal with the patches of upset and raised metal, so I'll be noting if anything is out of place. I don't really think the idea of loose debris causing the marks really works, because they're so regular, there are 4 marks, each perfectly (as far as I can tell just looking at it) 90 degrees apart (Doh, I thought I'd mentioned the angle, my bad). This regularity is what makes me think some kind of manufacturing defect.


any metal fragments anywhere?

I recall seeing exactly one small aluminium shaving, but this could _very_ easily have been cross-contamination from my workshop environment, the table immediately to the left of the motor is where my lathe lives. I did drain all the oil today and pull the sump cover and filter, nothing metallic there, though I did find some fragments of that hard-setting liquid gasket crap, fingers crossed similar pieces haven't got the lube system all plugged up.

[Dadpole]

"4 marks, each perfectly (as far as I can tell just looking at it) 90 degrees apart."

So much for my loose item theory. They look too rough to be a manufacturing "feature". Almost look like someone has used a puller in there - but it goes the wrong way for rotor removal. Very interesting indeed.

The clearance issue should not be a problem when the idler shaft is supported by the cover. The same system is used by most of the Suzuki road bikes.

[h0dgep0dge]

I'll post back with anything else I'm able to learn

[h0dgep0dge]

I'm pleased to report that I believe I have pushed the mystery back by one step. As it turns out I can't count, there are 3 high spots, on what it turns out is actually the driven part of the starter clutch. Those of you who are familiar with the GN250 might know the starter clutch has 3 rollers, and I've found that the spots are perfectly lined up with the spring-loaded pins that apply a preload to the rollers. It appears that some force has quite violently shoved the pings backward into the housing, causing these spots to poke outward, and then be ground away on the starter idler. The source of that force, I can't say. If I'm not wrong, the only force the starter clutch should see is a torque when the motor is starting, and this should not be applying force to the pins. I would even think that if something were to push the pins in, whatever was pushing would hit the casing itself before the pin bottomed out, so i'm stumped.

[Honest Andy]

Nothing to do with using the starter with the motor running is it?

[Dadpole]

That must have been quite some force applied there - or the housing is very thin.

The good new is that you know now. If the damage is not too bad then you can reuse the parts, else, replacements are not too much.

Even new (if you can trust them) - https://www.aliexpress.com/item/100-Brand-new-Top-Quality-Motorcycle-parts-One-Way-Starter-Clutch-for-Suzuki-GN250-GN-250/32235583349.html

[h0dgep0dge]

Nothing to do with using the starter with the motor running is it?

I don't know, I don't think so.

In the last few minutes I've also found another surprise, there are some nasty gouges in the crank case under the driving part of the starter clutch. It looks as if something got lodged between the starter clutch and the crank case, however there are no signs of damage on the starter clutch itself. This, along with the fact that I haven't seen anything more than a trace of metal fragments, makes me think it's an old injury. However, this could be related to the damage I've already found.

[Honest Andy]

makes me think it's an old injury

Never underestimate the stupidity of a previous owner...:(

[h0dgep0dge]

I just filed those spots off, and I've got that side of the motor reassembled. The screws holding the stator on were actually loose, I need to fire the mechanic who did that job *ahem* :sweatdrop

I expect it'll be at least a month before I'm back on the road, I'm in the process of tooling up to re-bore and build-up my worn cylinder head, wish me luck!