cleaning inside a hub dynamo

Have you tried cotton swabs, AKA Q-tips? If they don't do the job dry, perhaps moisten them with acetone. That would also help remove any oils that might interfere with adhesive bonding later.

You might also take a sharp steel scribe, or even a steel nail ground down to a point. Attach the strongest magnet you have to the shank. The pointy end will be magnetized and let you pick up particles in the corners. Maybe that will be sufficient. Then wipe up with an acetone-moist Q-tip.

I've got a small collection of small magnets. Some I've bought, but I've also disassembled old hard drives and saved their magnets. They're very strong, and affixed to steel mounts that can act as handles. Without those, I'd have trouble getting them off the furnace's sheet metal where I store them.

About compressed air: Before I had a compressor, I put an adapter on an old propane tank. I could fill it at a compressor (say, in the school's lab) and take the tank of air home with me. It doesn't last long, but sometimes it was handy.

I really wanted that idea to work, but no luck. Even with a good disk seek magnet the debris stuck in the dynamo. I ended up dry wiping the surfaces to be glued and crossed my fingers. So far it looks good. The clearance inside the hub is bigger than I thought, more than ten mils. Maybe it was much ado about nothing.

The hub runs smoothly and generates power, so I'll declare victory and retreat for now.

Next time the hub comes apart I'll give the sticky putty method a try. The debris seems to accumulate on the magnet faces and armature poles. A little oily residue will do no harm there.

As an aside, has anybody tried re-magneting a shimano hub? The factory magnet appears to be ferrite. It's reasonably strong, but NeBFe bar magnets should be a big improvement. The size would be about 30 mm long, 4 mm wide and no more than 3 mm thick. A quick check on Amazon found nothing that shape, let alone with the right field orientation.

Thanks to all who replied!

bob prohaska

Taking things in sort of reverse order: No, I've never heard of anyone re-magnetizing a Shimano hub dynamo. I remember Jobst had built a fixture to do that with the old Sturmey-Archer Dynohubs. Those would supposedly de-magnetize immediately if disassembled without a keeper. I made my own keeper to work on mine.

I'm curious about how a fancier magnetic material would work. I've thought about that for the old Dynohub, but it works fine for me as is, since this bike needs little beyond a "be seen" light.

But modern bike dynamos are usually engineered fairly carefully to self-regulate pretty well, putting out ~0.5 Amp and ~6V over a wide range of speeds. I wonder if a stronger magnet might ruin that self regulation.

I'm also curious about how your hub lost its magnet in the first place!