B31 engine and m20 frame

[watts]

Hello
I have 1952 B31 engine and a 1954 M20 plunger frame.  On first look it seems to fit ok is there anything I need to watch out for?
I think I read somewhere this is something BSA did do.
Any advice or should I source a M20 engine?
Thanks

[idie]

My ridged M33 started life as a M20, a PO had done the conversion before I had bought it. You may have to change the front engine plates. This will give you some more BHP.

[Derek996]

BSA produced the M33 which was a B33 engine in the M20 frame. The B31 crankcase is physically the same as those of  B33 and the M20 so it should fit straight in. You may have to fabricate a head steady though, between the lug on the cylinder head and frame.

[Pete C]

I'm not 100% sure but I think there may be clearance issues with the tank due to the B engine being taller...all that rocker gubbins.

Just been to the garage and the recess in the base of the B31 tank is definitely deeper than that on the M21.

[idie]

Unfortunately an M33 needs a special tank, it needs to be like a B31 tank but with M20 front mountings. if you try using a B31 tank on an M20 frame you will find that it sits to high and fouls the handlebar clamps. If 100% originality isn't a worry you can do as I did and weld two shaped pieces of steel to each the mounting ears on the B31 tank one inside one outside like a sandwich and drilled new mounting holes above the old ones. I painted the ears black so that they don't stand out. Of course if the tank isn't chromed and you feel brave you can just cut off the ears and weld them back on higher up so dropping the tank.

[watts]

Good, I will see how it goes and check tank clearance.
Thank you for the advice.