After some research, I still need some confirmation on B31 frame detail.
I am trying to locate a plunger front frame section with the larger steering bearing cups.
The smaller bearings were fitted to XB and YB (1945/47) and the larger cups from 1947.
Would I need the Z prefix frame, or do the Y prefix frame use both sizes ie
Was the change in the middle of the model run.
Any information would be greatly appreciated.
Thanks Keith A

email (option): keithatkinson@hotmail.com

Hi Kieth...As far as I know the larger steering head cups and cones, which were fitted previously to M Series models, were introduced to the B Series on ZB frames...So, this includes the late type rigid frame as well as the plunger type...

Frame fronts for the late rigid and plunger models are dimensionally identical and interchangeable, though there are some detail differences that mark them apart (if that matters to you)...

The early 'pre ZB' frame has the same cups and cone set as the Bantam. However, this is not the only difference between these and later frame fronts.

The early bikes used the 'B1' B Series gearbox which has narrower mounting lugs...In consequence the frame lugs are proportionally narrower as well to place the gearbox plates closer together. Also the rear frame is narrower where it attaches to the rear of the front frame under the saddle...

The later fronts and early rears (or vice versa) can be put together but it requires some modification....

You haven't mentioned what you are building...Is it a WB30 'lookalike'?..If it is the late rigid frame front is nearer visually to the WB30 frame overall than the plunger one...Ian

email (option): ian@wright52.plus.com

Hello Ian; Thanks for your reply.
I bought a B31 plunger rolling basket case many years ago, and I wanted to build it up, and also use up some other spare parts I have. The front frame section is XB and I have assembled all the correct parts in the steering head, including bearings, yokes etc. The rear frame (I think post YB) I have adapted to the narrower gearbox plates, and front frame, all with no problems.
I have a B33 engine, that I now want to use, being a little bit more powerful than the B31.
Now my idea is to upgrade the front frame section that uses the bigger steering head bearings, all for better overall strength.
I got hold of a YB frame, but the bearing recesses look too small, same as the XB. This is now confirmed via your email reply.
XB, YB and ZB yokes are dimensionally the same, except for the diameter and length of the stem tube.
XB and YB being smaller.
I am fairly sure the steering lock stops are the same. Other differences, if any, would not matter.
So I am building up a B31/B33 plunger bitza, but without cutting or permanently altering anything. I couldn't bring myself to do that.
Cheers, and thanks for your advice.
Keith A.

email (option): keithatkinson@hotmail.com

Hi Kieth...I think ,from memory, the early yokes (X/Y) have a smaller distance between the fork legs as well....Ian

email (option): ian@wright52.plus.com

This must explain my front axle issue

I found some replacement front tele forks, a super early type that are slightly shorter

my original axle now does not fit correctly, which was baffling me!!!

my r/h lower leg must take a shorter axle for the narrower early yokes i suppose??