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WM20 regulator box mounting

Hi all, especially the rivet counters amongst you,

A picky electrical equipment query: what's the correct way to mount the regulator box to the rear mudguard?

I believe the bolts are 1 1/32", pushed through from inside the mudguard, but I'm unclear what goes on the other side? Post-war models -according to my deeply unreliable Haynes manual- had a rubber grommet, (regulator box), washer and nut. However, an original WM20 mudguard I have has a more complicated arrangement of washer, nut, grommet, (regulator box), washer, nut...

I'm almost ashamed to be asking such a question, but it's also comforting to have the opportunity :upside_down_face:

Re: WM20 regulator box mounting

Craig, I don't know the exact 'correct' sequence but this earlier discussion has the parts listed for the pre-war set-up.

The only wartime parts list I have doesn't include the rubber packing pieces (rubber shortage I assume) so I think the earlier set-up with the rubber packing piece is the better way to do it.

I have a BSA Master Priced list that has a few extra dimensions that might help:

Bolt M253 - 1/4" x 1 1/32"
Washer B751A - .255" x .080"
Washer 27-8560 '255" (thin)
Nut M263 - 1/4" x .187"
Locknut EB102 - 1/4" x .109"
Washer 15-7383 (rubber)

This is the only picture I have that does indicate your suggestion of the bolts coming through the guard from underneath with washer and nut on the upper side is correct.


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Re: WM20 regulator box mounting

I've never been sure of the correct way either but thats pretty much how I do it, the problem is the mudguard shape throws the bolt tips so far out that the regulator holes won't go on, so you apply a little force which either bends the bolts or distorts the mudguard surface, you can use shorter bolts but then you can't fit the rubbers.

Perhaps the regulator isn't supposed to be easily removable and you were supposed to take the wheel out and fit the bolts from the outside, on Nortons don't you have to remove the tool box to fit the regulator?


Re: WM20 regulator box mounting

As Ray says, there is no rubber washers shown in the late war parts lists. I can't actually think of any other WD bike that has a rubber mounted regulator? Ron


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Re: WM20 regulator box mounting

Thanks Ron. The set up in your photo is the same as an original, seized up mudguard I have. Two's only a coincidence, but it makes more sense now. I initially assumed that the locknuts in my 1940 parts list referred to those holding the regulator box. However, seeing that these are the thinner nuts makes sense: they fix the bolts to the mudguard, before the reg box is fitted (with or without rubber washer between).

Re: WM20 regulator box mounting

Ian, I was looking closely at the photos I have collected today (trying to narrow down the various crankcase shields) and came across a good period shot of a voltage regulator fitted to a bike - a detail I hadn't noticed before.

This is a brand new M20 that has just had the original C number painted out with the Australian ARN number. Unfortunately that number is somewhat obscured but given the dozens of other bikes in the photo it is most likely from C910.

This clearly shows an unpainted nut holding the regulator tab and what really does look like a rubber spacer but that isn't clear enough for a guilty verdict.

It won't impact world hunger but an interesting data point nonetheless.

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Re: WM20 regulator box mounting

Am away for a few days now - but will send photos of reg mounting studs I have now found on 2 different mudguards
These were a one piece design with threaded studs either side
In the centre / there was a flat section where a spanner could be used to hold while tightening the nuts top or bottom end

These were fitted to later guards & I suspect these were a war time economy measure


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