dimensions of mainshaft threads

[seattle##gs]

AN part number 04.0373...clutch center nut. What are the threads? I think it's 5/8 x 20 BSC but I want to verify.

 

[seattle##gs]

I measured an old mainshaft...it measures 5/8 x 20. Looking for a tap I found many of these taps available. Standard UNF is 18 thread yet 20 thread is very common. I believe Norton used 20 thread for at least 20 years which implies that it would be a british thread and fairly uncommon. Anybody know why 20 thread taps and dies are so common?

 

[kommando]

BSC threads are nominally 26 tpi but at 7/16" and above 20 TPI was an allowed option and was used a lot.

 

[L.A.B.]

However, when CEI was adopted as a British Standard thread (becoming BSC in 1938) the 20 TPI series was apparently soon discontinued and cycle/motorcycle manufacturers used 20 TPI Whit Form so threads often identified as 20 TPI BSC might not be.





When entirely new parts were manufactured for the Commando the threads were commonly changed to Unified*, however, this appears not to have been the case where the Commando mainshaft was concerned as the part numbers of the two mainshaft nuts remained unchanged from the previous Dominator/Atlas parts.

*(5/8" x 20 TPI Unified would be 20-UN)

 

[seattle##gs]

Thanks for the info. I found a 5/8 - 20 die in my selection of rarely used tools and lightly threaded it on to the mainshaft. It went on easily. I remember buying this eons ago to clean up some battered threads. My interest in the tap is because I want to make a tapered piece to screw onto the end of the mainshaft and make it easy to install the clutch circlip 06.0752. After a few decades of trying to work the circlip onto and over the splines with a pair of tiny screwdrivers, a process that usually takes several tries and much patience, I think I can make this a quick and easy job with the right tool. It's possible that I'm over engineering it by requiring threads. It might work just as well with no threads.
I am in the middle of what seems like an almost impossible gearbox problem - very stiff shifting and rattling it into first gear every time - which has meant R&R the gearbox internals several times. There is a high level of frustration but at least I can overcome the circlip headache once and for all.

 

[possm]

The pitch of the thread is different I believe BSC is 60 degrees where as Whit form is 55 degrees.

 

[milfordite]

Re: the gearbox, check all shafts for straightness, including the rod the shift forks slide on. I've seen bent rods, bent layshafts, and bent mainshafts.