Charlie's Shed - W/NG Dynamo Repairs

 Some while after fitting the new headlamp and instrument panel, I became aware that the ammeter rarely showed a charge. At the time, I assumed it wasn't centring properly and resolved to pull it out and adjust the needle. However, obsession being what I'm known for, I kept an eye on it and eventually realised the needle intermittently showed a charge. There had to be something wrong.

Following the usual checks (voltmeter on the battery, linked D & F cables with voltmeter to earth, connectivity checks) I realised the dynamo was struggling to go past six-point-something-small volts.

I went inside, and here is what I found:

That's it - goop and copper dust. I sprayed it with brake cleaner and put it back on the bike, with the brushes now sliding in their holders. A few trips showed a steady improvement, but it still wasn't right. Waiting until I had a few hours I could dedicate to it (it's in daily use, more or less), I took it off again and stripped it down to find the shims that one of the previous owners had fitted (and I had re-fitted) to accommodate the non-standard bearing had moved and cut into the commutator, shorting out the segments intermittently.

I resolved, after much deliberation, to bring the bearing arrangement back to standard. As part of the process, I cleaned up the commutator on the lathe:

The bearing came off after a few minutes with a grinding wheel in the Dremel. It wouldn't budge for a puller, but the Dremel cut through the outer race easily so that I could split it and pry off the ball cage without damaging the segments. I ground the inner race tangentially until I was almost through to the shaft, when it cracked and came free. The shaft was in good condition underneath, measuring a hair under 7 mm - odd, because the Lucas parts book suggests the internal diameter of the bush at this end is 8 mm. This must have been enlarged post war - this dynamo is marked 1943.

These are the culprits - two pieces of thin steel:

I made a drift to knock out the old bush:

I turned the new bush out of a bit of SAE 660 phosphor bronze I had. This is a bearing bronze that is considered the standard bearing material for light duty applications. It is easily machined and the bronze alloy is hard, strong and resistant to wear. Apparently it also conforms well to journal irregularities and is less dependent on lubrication than other alloys.

I drilled it 6.5 mm and reamed it 7 mm:

The old bush measured 11.2 mm outside diameter and pushed out easily. I made mine 11.3 mm and whilst it went in with no trouble, it was considerably tighter than the old one. I reamed it again when I had it fitted and deburred the ends.

The clean armature journal fitted beautifully and I reassembled the dynamo, pausing to look at the drive end which seemed to have a lot of end float, though in fact it was around 0.5 mm.

I attempted to motor the dynamo on the bench, but unfortunately the only loose 6V battery I had was completely flat. I refitted the dynamo to the bike, and started the engine to find the ammeter kick over to the 'charge' side with a few revs. Job done, except for the felt washer around the bush which I don't have at the moment.

Another small job I wanted to do was bush the end cover retaining screw hole (3/16" BSW) which has stripped. Unfortunately there is not enough room to fit my preferred M8 bush, which is plenty big enough to tap 3/16" or even 1/4" BSW if you have to, so I re-tapped the hole M5 which is a little bigger than 3/16". I'll have to think about that some more - perhaps go for an M6 brass bush which I can drill and tap 3/16" BSW.