Happy new year to all.

I bought a BSA frame on ebay with no numbers in the auction. Now I unpacked the well-wrapped frame and immediately looked for the number. The front frame is marked KM20 1411. So far so good, but the rear section is not an M20 part. I know these frame dimensions from my M24. An SV Engine does not fit into this framework. I suspect the rear part is from an M23 or some other OHV engine. Any other guesses ? Is this M20 Frame delivered to WD ?


cheers Klaus


https://ibb.co/SBPj3RX

email (option): ajay2@gmx.de

KM20 1411 was an M20 D/L despatched to the War Office on 5/5/1939, fitted with an engine bearing the same number. Colour was listed as 'Green' The War Office contract number was C3139.

Hi Klaus,

Your rear frame is a pre 1938 M model frame. The 1937 M20 engine only had 1 bearing on each side of the crank. The crankcase itself was less wide then the later M20 engine. The 1937 rear frame was less wide as well. Besides that the shape of the lugs for the pillionseat footrests are typical pre war/1937.

Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

Bastiaan did you mean 3 instead of 4 bearings?:wink: Ron

email (option): ronpier@talk21.com

@Ron: yes, sorry you are right! 3 bearings for 1937, but I know the frame is less wide.

Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

Yes they introduced the extra bearing on the timing side in 1938 I think, and created a cooling air gap behind the tappet chest. Which made the engine wider. Cheers Ron

email (option): ronpier@talk21.com

You are right about that Ron. I wonder if the modification on the cylinder barrel was carried out based on the conclusions in the evaluation report of the MWEE after the tests in 1937. In one report the following is stated:

"Both machines developed irregular firing when under load.

Results of examination:
The defect is identical in each machine and may be summarized as "sticky" valve stems.

All valve faces have been damaged through overheating and machinery will be necessary. The seatings need grinding only. "

email (option): wdmotorcycles@gmail.com

Thanks everyone for the helpful explanations.
cheers Klaus

email (option): ajay2@gmx.de

AFAIK, all the prewar O.H.V. M-type rearframes had the tube around the oilpump crankcases bulge.

Believe the pillionlugs changed from straight to angled around 1939

There are also a few variants in the piece under the saddle that connects both sides, that might give a definite clue about the date.

Best Wishes!, Michiel

Hi. Really interesting frame. According to my experience all 1937-38 M model rear frames have threaded holes on both sides of distant piece exept M24 Gold Star. Purpose of the holes is unknown to me.



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Believe the threaaded holes are a leftover from earlier models where is was used to attach the saddle springs and/or the rearcarrier. BR Michiel

Klaus, does your frame have the threaded holes Kalle refers to?
Lionel

email (option): Lioneljrudd@gmail.com

Hello Lionel,
no there are no treaded holes. It´s all solid and braced.

cheers Klaus

email (option): ajay2@gmx.de

Think the crossbar should be straight as the whole distance/connection piece including the balls is one part made from solid by turning.

BR, Michiel

Hello Michiel,
I don't think so, the frame looks really little used. It would also be difficult to assemble this as one piece.
If the spacing of the frame is correct, I can measure it, then it's probably correct the way it looks.

cheers Klaus

email (option): ajay2@gmx.de

Kalle,

No all frames in 1937 had those threaded holes. I have here an original complete M20 frame from 1937 which is lacking those holes as well.

See also the photo below of one of the War Office M20 prototypes from 1937, it doesn't have the holes either.



Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

Hi Bastiaan. It can't be remain from earlier design then. I have 2 frames with the holes. One is 1938 for sure, the other is most likely 1938. It would be interesting to know the function of these holes. Thread is 5/16 BSC

Kalle, why can't it still be a remain from earlier design, than?

Most 30's models had this type of design, and again, in earlier models it was used for taking up the saddle springs and/or the rearcarrier.

In the M type these threaded holes had no use as they had seperate lugs for the saddle springs.

Also convinced it is a solid/one piece connection, most likely from a lathe, and it should be straight as I have never seen a curved one.

Maybe other members with some knowledge of the earlier prewar models can give their views, please.

Br, Michiel

Hi Michiel. Of course it can. But why to delete this part from some 37 models and then put back for next year. I agree with you that it should be straight. Very easy to bend it when rear frame is loose as this part only connects two sides of the rear frame.
One more quess is fixing point for high level exhaust.

Hi Kalle, yes, sorry, your correct, it could have another reason which we don't know (yet).

I looked in the 1939 catalogue and to my surprise all the 3 versions seems to be illustrated.

M20 standard : nothing to see, so probably a tube like the later models.
M20/M21 deLuxe, M23, M24 : ball with threads like your frame.
M22 : ball but without threads, like Klaus his frame.

I don't know....the high exhaust seems to be fitted to the saddle spring framelug, and the saddle spring itself seems to block fitment of a rearcarrier...

Br, Michiel

Klaus, just noticed the 1939 M22 rearframe is exactly as yours, it looks to have the balls without the threads and also the straight pillion footrests.

In 1939 the M22 still had 2 triangular toolboxes on both sides, placed high and vertical up the mudguard; on the other M models the single toolbox came between the chainstays in the position we all know and this needed a change in footrestlugs.

Br. Michiel

Hi I have complete "37" M20. That is an early "37" rear frame section. Based on that frame number, front part of the frame is from "late 38-39" m20. My "37" does not have the threaded holes. There were at least three variants of m20 made in 1937 with slightly different engine and frame parts. I have seen parts from the other two variants. Charlie

email (option): millcrocfarm@gmail.com

Klaus, looking at the drawing and noticing more variants than I thought, I am not so sure anymore that in your frame the connection needs to be straight instead of curved like it is.. :relieved: :innocent:

Michiel, 1939 M22 you posted was just repainted and restamped KJM22 that was left over from 1938 model row. Later they produced small number of real KM22's with new 1939 engine and frame design. I have seen at least one KM22 stamped engine.

This forum is really great with lots of knowledge and information. I think we can assume that this type of frame was built until 1938. Except for the M24 which had no sidecar connections and frames made of Reynolds tubing.
It's amazing what details come to light.

cheers Klaus

email (option): ajay2@gmx.de

Thanks for all the info in this thread. I have front frame HM2398 on the bike I am building up with the later rear end.
Now I have to find a new rear end. The hunt goes on. Not too many early model M20s here that I have seen.

email (option): Lioneljrudd@gmail.com

email (option): wdmotorcycles@gmail.com

Hi Bastiaan
How is the front end of the rear frame designed to take 39 wider engine? Is it a hybrid from the both types?

Kalle