How does it work? How hard is it to rebuild? Where can parts be found? Is there an expert repairer?
Work? Not very well, output is about 20 watts maximum. In countries (like Edin's) where they require daytime running lights, even for vintage vehicles, the dynamo just can not keep up.
Rebuild? It is a permanent magnet, four pole dynamo, so is quite simple in construction. Short of the armature or commutator being damaged you are looking at replacing the brushes and if needed, ball bearings. It is recharging the four-pole ring magnet that is the problem. No one wants to touch them as they are likely to make them weaker rather than stronger. You can not charge one set of poles and then the other, they both need to be done simultaneously. That requires special kit that was likely last seen at the BTH factory! What needs to be done is special pole pieces made so it can be done on a standard magnet charger. The reward for all this trouble is a full and blindingly bright 20 watts output.
Parts?- Nonexistent since the mid-thirties. You collect other BTH pancake dynos and hope for a stronger magnet. Lucas brushes can be filed down to fit.
Experts? Nope, everyone looks after their own. Most have been removed and a blank-off plate installed!
But if you like to ride your bike a lot, then you do what you need to, or have several swappable batteries on standby!
-Doug
Addendum- I have been told the BTH Pancake dyno was used on the early LE Velocette, continuing availability into the early forties.