What is the Camshaft thread for Commando?

[thunderbolt]

The camshaft nut on the camshaft on my '74 MK 2 is quite tight. I have purchased a new nut and it is tight as well. I have been able to gain some info off the RGM site that the thread is 3/4" and 20 TPI. It does not say what the thread form is: BST, UNF, etc.

Does anyone know the thread form used and the best way to clean up the thread so the nut will be a good fit. I do have a thread file with 20 TPI but I don't know what the included angle of the thread is (60 or 55 deg).

Any suggestions greatfully accepted.

Thanks

Don

 

[guest76]

If memory serves me right, it's 20TPI coarse cycle thread (BSCy/CEI), and is the same nut as used on the clutch.

Thread angle is 60 degrees

 

[SteveA]

If memory serves me right, it's 20TPI coarse cycle thread (BSCy/CEI), and is the same nut as used on the clutch.

Thread angle is 60 degrees

You memory is tricking you! The clutch nut is 5/8" 20 tpi, same as the end of the crank thread, but a different nut altogether.

Cam nut is 3/4" x 20tpi, and I have a reference that says UNEF, Unified 'Extra Fine'!

See lower part of the chart in this reference to 3/4" 20 TPI https://en.wikipedia.org/wiki/Unified_Thread_Standard

When I purchased a PW3 a standard AN nut would not run on, as a compromise to changing it when I really didn't have time for that the supplier chased a nut to fit!

And as an aside, if this nut has to be torqued to 70 lb ft to stay tight on original cams, why does the same nut on a chilled iron PW3 stay tight at 15 lb ft?

Still struggling with that as I decide what is the best torque setting for my now fitted Web cam.

 

[thunderbolt]

Thanks guys for the replies,

I tried the thread file (20 TPI) on the camshaft thread and it was too hard for the thread file to make any impression on the thread.

So all I could do was use some grinding paste and work it back and forward till the thread loosened off a bit between the camshaft thread and the nut. It's not perfect but much better than it was originally.

Thanks

Don