Atlas lower gearbox stud nut size?

It’s most likely to be cycle thread . I bought a mixed bag of cycle nuts and bolts about 25 years ago and have lost count of how many times I have dived in there to replace missing ,stripped or just rusty items  , it’s a no brainier ,you will not regret buying a bag . What year is the Atlas? Some threads changed on late models. There may be a list of thread sizes in one of the Dominator compendiums on this web site.

The lower Atlas pivot stud is part number 16410 which is the same as the 650ss. This is 1/2 x 20 x 4-3/4 inches. The washers are two of them 000008 which are 1/2 x 7/8 x 2mm. The two nuts are 041265 which are 1/2 x 20 inches. 

Like Robert said - It is more than likely to be 1/2 inch BSCycle thread 20 tpi.  On my 68/69 Mercury however it seems to be 1/2 inch UNF thread 20 tpi.  When Commandos came along in 68 quite a few threads changed to the UNF format.  The exception were threads on the engine. These mostly stayed in-line with the Dominator/Atlas convention of BSW, BSF, BSC.  Some of the electrical threads changed to UNF and some of the ex-Smiths threads stayed as BA format.  It can be confusing.  The later Commandos were mostly UNF/UNC threads.  Good luck, Howard 

13.5mm gives an equivalent Imperial size of 0.5315 inch. So is that the size at the threaded sections of the stud or the shank ?  Cheers, Howard

RGM list the bottom stud complete with two nuts as 1/2" UNF (exactly as Howard says), which is 12.7mm on the shank.

 On an earlier bike it would almost certainly have had cycle thread.  I think you'll be lucky to find a single 1/2" cycle thread nut, so you might have to buy the entire stud with its two nuts.  The only issue is that it will need an "AF" size spanner, unlike the rest of the bike.

Hi Graham , I probably have a spare rusty nut. Where are you based?.

I live not far from biggin hill if you are comming thro, I will be at Ardingly on Sunday .

He'll be the one in a Norton t-shirt. Oh, wait a minute... 

This stud in your 66 Atlas will be .5'' x 20TPI x 60deg. It is BSC and UNF. .5'' BSC can be either 26 or 20 TPI. 

It might be that a previous owner found a badly worn hole in the gearbox. Or in the engine plates.  Made an oversize stud to fit. If you has measured diameter correctly to be 13.5 mm. Possibly made custom nuts too. In that case he could have made an imperial or a metric thread. Depending on what lathe he used. Closest thread diameters to 13.5mm I know of is 1/2" at 12.7, BSP 1/4 at 13.157 and 19 TPI and M14. If the normal 1/2" x 20TPI BSC don't fit, sadly you have to remove the stud in order to use a thread gauge to figure out what it is.

As the stud is quite awkward to measure in place, you could use different open spanners as gauges to figure out diameter of the stud. And check spanner size with a vernier.

I will be on the NOC stand , If i am off gawping at other stuff the nut will be wired to my carrier ,red 99 with panniers. BOB .Hope its the right one !.