B31 rigid-plunger

[V500]

Can anyone tell me if rigid and plunger engine/gearbox plates and brake pedal are the same?
Can the plunger crinkle hub be adapted for use on a rigid?
Thanks.

[idie]

There is very little difference between the rigid and plunger frames. So I would quess that they would be the same
 I have used a crinkle hub from a plunger on a swinging arm frame
 It appears that the rigid hub and the swinging arm one is the same
 To use the plunger one you need to change the bearing spacer and cut off the fine thread on the offside of the hub if you want to fit the cover plate. I cut mine off by mounting the hub in the sprocket and running the bike in gear on the stand and holding a hacksaw against the hub.

[Derek996]

The engine and gear box plates are the same.The plunger and swinging arm frames both use the crinkle hub. To fit the crinkle hub to a rigid frame, as well as the changes to the spacers you would need to open up the slot in the frame to accept the bigger diameter spindle and ideally make some arrangement for adjusting the chain tension and maintaining the wheel alignment.

[idie]

The rigid frame will have two adjuster bolts for alinement just change the spacer to one from the swing arm then you can use the correct spindle without opening the frame slots.

[Derek996]

The rigid frame will have two adjuster bolts for alinement just change the spacer to one from the swing arm then you can use the correct spindle without opening the frame slots.

Yours must be a very different rigid frame from mine. There are no adjuster bolts for alignment, this done by 2 snail cams, one either side.

[idie]

You may well be right. The one I had, I found the bike rotting in a garden where it had lain for 50 years.

[V500]

I seem to recall my old Empire Star had that snail cam arrangement too.
It would appear that the plunger brake brake pedal is mounted on a boss on the frame and uses a through bolt to secure it, while the rigid frame mount looks totally different in this respect.
Any thoughts on compatibility or not?
Thanks for the info so far.

[Derek996]

The rear brake pedal is mounted on a pivot pin that appears to be threaded into a boss on the frame and has a nut and washer on the outer end to retain it. Here is a picture of the rear section of the frame of my ZB32 Competition.

[V500]

This is supposed to be a 1947 B32 Comp bike too, but the pedal mount looks very different.
Any thoughts? Modified, different year, whatever?

[Derek996]

My bike is a '48 with the Lightweight gearbox. The same part number is listed for the '47 and 48' frames but changes for '49 with M20 type gearbox which is ½" wider across the fixing lugs.For what it's worth here is a picture from the 1947, '48 and '49 parts lists. The same frames and chain stays are listed for the B31/33 and B32/34 models.

[V500]

The frames in the diagram would appear to match your frame, but not the one in my pic.
I wasn't aware of BSA fitting the lightweight box prior to the later M20 style box. Would you happen to know the lug dimension for the lightweight box?
Thanks for your help.

[Derek996]

The lighweight gearbox is 2" across the lugs whereas the M type gearbox is 2 ½" hence the different engine plates and rear frame section. You will find a lot of information about both gearboxes here: BSA B range gearboxes - and what mods are needed to fit the later gearbox to the early frame.

[neil1964]

There is a rigid B31 hub on eBay now

[GLENN]

@v500, your frame looks correct for the 47/48 rigid frame B31.  I can't confirm if that's the same as the competition frames, but if they use the early/narrow gearbox I'm guessing they would be the same.  I have 2 of the 1947/1948 rigid B31's here if you need anything specifically checked? 

Best,

Glenn

[V500]

Thanks very much Glenn, much appreciated. Do you reckon the brake pedal boss on your frames match the one in my pic?



There is a rigid B31 hub on eBay now
Yes, I saw that thanks, but it's bare and surprise surprise, Drags have the spindles on back order.