Dommie twin-carb to head stud and nut sizes

Hi James,

I cant speakspecificallyabout your 1960 M99, but my '57 Model 77 (600cc M99 engine) has 1/4" cycle threads on the nut end of the studs. Unless a change was made between '57 & '60, it may bepossible that someone put the wrong studs in? I am fairly certain that the end of the stud with the nut is 1/4-26 TPI for your bike, but the end in the head is 1/4-20TPI (?). I am not sure of the TPI in the head, but I know that once Norton changed from an iron head to an aluminum head, the stud went to a coarser thread in the head to prevent the fine threads from stripping out. With that in mind, if someone replaced the original studs (coarse on one end, fine on the other), with a stud that had coarse threads on both ends, that would explain your troubles.

I show the correct part number for the studs is #18380, on Norvil's website, they have a nice picture of the studs. Norvil sells the 4 studs in a set with the nuts for 9.77 GBP. Here is a link to the picture:

http://www.norvilmotorcycle.co.uk/018380a.htm

Skip Brolund

Hi.

To add to Skip's advice - Norton used many 'small-hexagon' nuts on their bikes. When you say 'they don't fit' do you mean size-wise or thread-wise? Chances are (if you mean the studs that fix the manifold) they may have been 1/4 cycle thread with a 1/8 WW spanner size (as shown in Skip's link) instead of the normal 3/16WW spanner size. My 58 Dommi's twin carb manifold was fixed by 1/4 Whitworth threadhexagon headed screws - the heads were 1/8 WW spanner size. The Carb stud nuts - Part No. E3231 - were 5/16 cycle thread with a 'small hexagon'3/16WW spanner size as opposed to a 'normal' hexagon of 1/4WW spanner size.

Ian.

I would try some nuts. My guess is that somebody has replaced the studs, and they may well have made ones with a 1/4-20 UNC thread on one end and a 1/4-28 UNF thread on the other. UNC and UNF threading tools are easier to find these days than BSW, BSF and BSC ones. Try a 1/4-28 nut and see if it fits.