What is this BSA Frame??

[peteR]

Can anyone shed any light on what this frame is?

The engine is a 1952 M21 that has some AA history. I can see that someone has fitted bantam forks to it, but I dont recognise the frame at all.

The frame number is not legible.

The engine is not a good fit into the frame, but am trying to assist my friend identify what he actually has.

If you need any more pictures or info, please let me know.

Cheers

[Rex]

The rear plunger section is likely from an A series or B series BSA, but as for the rest?

[R]

1920s BSA's had an I-section backbone, but I'm not sure if it was like that.
Something about Francis Barnett's "built like a bridge" comes to mind also,
with all those bolted joints ?

[cardan]

Hi Pete,

What a ripper! The main frame is certainly BSA, very likely from a vertical-engined model with the oil in a "sump" out the front, early-mid 1930s. The best plan is to decipher the frame number - on the forged steel backbone somewhere - then have a look at a list of BSA frame numbers to see what comes up. https://www.bsa1930s.nl/engine-and-frame-numbers/

The rest of the bike has been "added" to the older part of the frame. Worth leaving as an interesting mix.

Cheers

Leon

[Ed: took me a couple of goes to spell decipher!]

[cardan]

This sort of thing - this one is a 350 from 1936 - but there were lots of variants.

[john.k]

Its not surprising the motor has been replaced ......the mid 30s BSA singles were not a very good motor ,in fact they were crap .......a very poor effort from such a big maker............which is why they brought in Val Page to redesign all the BSA singles to be decent bikes.......Yes ,I had a couple of 1934 Blue Stars ,good gearbox ,crap motor.

[john.k]

The motor leans forward because the original motor had the magneto at the front ,with space at the back of the crankcase ...........the mega dollar BSA ohv V twins use the same frame backbone ....J 12 and Y 13 ...and sell for mucho dollars

[peteR]

Thanks guys.  I have managed to find the frame number hidden away.

The frame number B3-593 from a 2.5HP 1934 BSA Blue Star 250c.

Agree that the plunger section is probably from an A or B series.  The forks actually seem to be C15 and the front wheel is Bantam D1 with the smallest brake imaginable!

Other than that, it seems to be just as it left the factory!

[cardan]

Hope your friend wasn't expecting more... a bitza can be fun but very disappointing if it's a surprise.

Leon

[john.k]

The toolbox is correct for the plunger frame ,its odd its mounted cockeyed .........BSA had the forge shop to make the frame ,a part of their heavy weapons plant..........BSA also drop forged the flywheels ,even the lowly side valves had forged steel flywheels........in fact you can make a very capable OHV from a BSA M20 or M21.

[peteR]

Hope your friend wasn't expecting more... a bitza can be fun but very disappointing if it's a surprise.

Leon

Thanks Leon, I think he always realised that it was neither one thing or the other, but the extent of the variety of models included has come as a bit of a surprise.  Interestingly, the log book shows it was first registered in 1934 as an M21 (i.e. 2 years before Val Page joined BSA and designed it!)

P

[john.k]

Ive collected lots of M20 and M21 motors to make a Gold Star style OHV  motor and fit it into an early Kawasaki frame I have ........but ,at my age , reality has finally struck home ......no more bikes for me ....enough already.