Shimano DH3n20 dynamo no power

[Brucey]

It is a right-hand thread, turn clockwise to tighten. So was my Shimano T785 dynamo hub, as was an older cheap Sanyo hub that I opened up, built just like the Shimanos, with a threaded cap on the side. I'd say they are all right-hand threaded.

if the cap screws into the hub from the right side it has to be a RH thread to resist precession. If the hub was similarly constructed, but with the output on the left, the cap would need to have a LH thread.

[Brucey]

....I might try straightening with a socket or something......

I'm sure it looks dreadful but it might yet be a relatively easy fix. I'd probably tackle it as follows;

1) remove the steel seal part from the cap.
2) temporarily refit the cap in the hubshell.
3) choose a 1/2" drive socket that will just enter the hub (so about 33 mm OD then).
4) secure the socket in place using M8 or M10 or M12 studding + nuts etc
5) massage the dent out with a little hammering; change to a larger size of socket when you can.

The steel piece will also need to be reshaped, but this ought to be much easier. The cap is probably a die-cast part; modern die-casting alloys are usually very ductile, so it might be easier than you expect.

[Dr pepper]

Ok good idea bolting up a socket, I have repair washers and bolts for that.
I'm not so sure about taking the seal cup out, it fits up to a register so I'd need to tap something in behind it and that might bend it up more.
Looking closely at the cap you can see that the bent section of the cap and the seal cup is higher that the actual seal surface of the steel seal cup, so Im thinking I'd be able to straighten both at the same time, at least at the top which is where is needs to be straight.
I'll have another look tomoz, if I need to remove the seal cup I'll stick the cap in hot water to loosen it.
Off for a ride now, on another bicycle of course.

[Brucey]

I have removed seal rings from many shimano hubs and whilst you do need to be careful with them, they usually come out without any fuss. Because it is just the cup behind, you can normally fit something 2-3 mm longer than the hole is wide through the hole so that it can be hit from behind. I think you can make a super tool for this, using an old spoke, if you go about it in the right way.

[edocaster]

Good job getting it off. It's really easy to marr the flats and warp the metal, as you've found. Did it happen after removing the cone and bearings? It may have been better to have them in place.

All looks very uncorroded as well.

[Dr pepper]

Sound suggestion about having the cone and bearings in place, they'd have to be loose of course to allow movement of the cap.
Well it didn't work out so well, building it back up the cone rubbed on the octagon, so I tried another little tap with the socket to shift it back into place, broke one of the octagon flats off, oops.
Not so ductile after all, before I did this I drilled a small hole at the end of the crack as it wanted to go along the surface of the cap, thanks to this it did not and it looks like theres no major weakening of the cap except for the missing piece, the seal cup is on show, there may be some means for water to get in behind the seal cup but I think it'll last the summer.
I now get 40 volts ac when I spin the heck out of it with no load.
Fitting the seal on the left side was tricky, I put the seal in, then put the cone in, and tightened, shifted the seal, and repeated till it was all the way in. Thanks for the help people.

[Carlton green]

My thanks to the OP for his updates and sharing the outcome; it’s a courtesy to those that have offered comments and an aid to others with similar problems.

Looking on eBay I see that there is a suitable (octagonal 36mm A/F) ring spanner available from Germany for about £20, not cheap but probably worth it. I’d wondered just how malleable the casting would prove to be, thinking that sometimes such cast parts were made of ‘monkey metal’ (a historic and derogatory term).

I realise that they’re rare but experiences like this one confirm my own dynamo preference (for the traditional design). In many ways my choice is inferior to hub dynamos. However the traditional design is functional enough for my needs and should one break then it’s a ‘five minute job’ to unbolt and replacement cost is about a tenner; in contrast the cost, time and disruption of rebuilding a hub dynamo wheel is not insignificant. … puts on tin hat, retreats and then disappears .

Whatever, thanks again for the update and good luck with the damaged unit.

[Brucey]

.....I tried another little tap with the socket to shift it back into place, broke one of the octagon flats off, oops.
Not so ductile after all.....

oh dear; still, it's nothing that can't be fixed with some epoxy resin plus some SG action. If you include some Al filings as filler, the epoxy resin might even colour match nicely.

[Cyclothesist]

It's been an informative thread. Thanks for sharing with pics. I was hoping Brucey would come along to say how the hub cap can be fixed. Is 'metal putty' such as JB Weld an off the shelf option? You have at least got a functioning dynamo hub now and the damage is largely cosmetic.
If you do repair it with epoxy filler you can probably get an acceptable cosmetic result painting the octogon with silver nail polish.

[Dr pepper]

Gad you guys found it useful, I keep posting for that reason and as an appreciation for those that help me.
I have some metal epoxy putty I've use for repairing bearing housings on industrial stuff in an emergency, I might try some of that, if its the last time I work on the hub it'd probably give another years service, which would be enough.
I made the octagon tool from 6mm ally treadplate, it didnt even mark the hub cap paint, if I'd have used this from the start you wouldnt have know I'd been messing with the hub.
I'm toying with the idea of changing the shimano for a sturmey archer dynamo / drum brake, I know they dont brake as good but I'd loose the 'eeek' and the perpetual warped disc shudder every time I pull the brake lever.