The BSA Workshop > Singles

B31 crankshaft/ cushdrive woes

(1/3) > >>

I'm building a B31 from parts, I have a 2 lobe cushdrive but not the retaining nut/collar to screw it to my crankshaft. The problem is I've discovered that threads where it screws on, measure 11/16" on my crankshaft, whereas my pals crankshaft measure 3/4" (Draganfly list this size). Does anybody have any thoughts on why my crankshaft thread is a different size? It has the 66 610 casting stamp which I believe is normal, and it fits into the bearings and cases fine. Is there a smaller size retaining nut/collar?
I could look for a complete cushdrive but I'll still have o find a nut for the smaller crank thread whichever cushdrive I use.
All suggestions welcome.

It looks as though the 1953+ B31's used cush drive nut 42-71 which has a 3/4 x 20 tpi thread. However, earlier B31's used cush nut 65-2520 but, unfortunately, I can't seem to find what thread this used. It's possible this used a 11/16 thread?  It seems most B31 crankshafts were stamped 66-610 but do you know what year the engine is dated or is it just a box of bits?

Hi Arthur
It's a box of bits I'm afraid. Shaft thread seems to be 22 tpi, if I'm reading my gauge correctly. I had thought if all else fails I'd find a nut and weld a big washer to it, but as yet, I can't find any reference to 11/16 22tpi.

Have been through the old parts books.

The B31 used the same drive side mainshaft from 1946 until 1957 when it changed for the alternator engines.  Part 65 1128 for the shaft and nut 65 2520 until 1955 when it changed number.

So the thread would be 3/4 inch x 20 tpi cycle for both cush drive nuts.

I dont recognise a 22 tpi thread. A cycle thread would be 20 tpi or 26 tpi.

22 tpi is fairly close to a 1 mm pitch metric? And 18 mm is close to 11/16?

22TPI is nowhere near 1mm.
1mm translates to 25.4TPI which is close to 26TPI


[0] Message Index

[#] Next page

Go to full version