Douglas did use Whitworth coarse threads in cases where they were tapping into aluminum (or aluminium). The ohv engines use 5/16-18  for cylinder studs. Both ohv and sv engines used the same thread for magneto hold down studs into aluminum. The 350EW, B29, L29, L3, H3, C32, A31/32 used 1/2-12 for the crankcase mounting inserts (more on that momentarily). The 250/350 Aero used 9/16 Whitworth for the crankcase mounting clamp bolts. In the cases above factory drawings state "1/2 Whit", "5/16 Whit", or "9/16 Whit" as the case might be. However drawings for 3-speed gearcases for the 350EW, 600EW, and the S5/S6/T6, are annotated just "3/8-20". No mention of thread series such as Whitworth (fine) or CEI/BSC-20. And contrary to the aforementioned practice, and good engineering practice, of using a coarse thread in soft, ductile materials. It would be a good experiment to inspect the thread profile of some original gearcase bolts.

Other exceptions to using coasre threads in aluminum are the cam spindle outrigger plate bolts for things like the A31/32 and the Aero series, which used Douglas' own 5/16-25 thread. Timing cover studs and screws were frequently 3/17-27. These though do not tend to be torqued as high and crankcase and engine mounting points.

In situations where a strong purchase was required as well as the expectation of removing and installing the fastener multiple times, Douglas would fit a threaded insert. They did not trust the wear properties of their own aluminum! Given that the bolt or stud is threading into steel, the above premise does not apply.

According to the EW spare list, they mention both an Anchorage Bolt (7596) and Anchorage Stud & Nut For Stud (9251 & z/467) So a bit confusing as to if it should be a bolt or stud, maybe both were used at different times. The anchorage bolt for the gear case is 7741. Had it been the same number as the engine mounting bolt, then checking the profile on a gear box bolt would be I think 99.9% conclusive. As it is, I think it would be 99%. Nut z/467 is also 3/8-20 and dates back to the 2-3/4hp era. Those threads that nut is used on that I have checked had a 60 degree thread profile.

-Doug

Tell us how it goes.
From the deafening silence here, its almost like no-one has ever removed these studs to
see/say what they are ??

The Owners Club seems to like keeping it secret what spares they hold too,  which is a bit
unfortunate for bikes being restored - so is it worth joining at this stage in proceedings ?

You could - very carefully - try to thread a 3/8 BSF bolt into the threaded crankcase holes.
If you meet any reluctance - whatsoever - cease and desist.

I have noted that manufacturers, if they don't generally use BSF, will go to any lengths to avoid using them. !
Inc using the aforesaid 20 tpi coarse cycle thread.
Always a possibility ....

Note well the comment
https://britishfasteners.com/threads-bsc
"We have numerous taps and dies in BSC with 20 tpi threads."
"Angle of thread = 60°  "

Hi EW Ron…thanks for going to the trouble of removing the stud for me. I can get both the nuts and the studs here locally.
Take care.
Matt.

Eddie,

many thanks for your helpfull advises.
I have now started stripping down the bike, but must find someone for TIG welding.

I thing about the construction of the early frame type (MK1+3).
There is above the the swinging arm lug the small wellded / brazed? on triangle to bolt on the back frame.
This brings also negative side pressure to the frame tube.
May be, it is good to enlarge the triangle down to the lug?

Michael