The screws to fit the Doherty square edged air and ignition control lever housings on an early Norton are definitly 3/16 x 28 tpi, they are not UNS 10-28 and also not 1 BA.
I had to make them and used an old fashioned die wrench with loose 1/4 x 28 tpi UNF cutting plates. Brought dia to 4,8 mm and cut the thread with that.
They fit perfectly into the control housings, as well as in the throttle control clips.
Thread angle is identical to the single original screw I had, checked by fitting the threads together (eyeball Mk 1 observation).
Likely a Doherty attempt to assure delivery of spare parts by using proprietary screws.
Other brand levers may well have been 1 BA, I doubt 10-28 UNS was ever used in the British industry of the time.
The UN definitions came when threads were universally specified in 1949.
Likely a Doherty attempt to assure delivery of spare parts by using proprietary screws.
Here’s another oddity: the 6 bolts that connect the Mansfield pillion seat frame to the O legs (or X legs as well…) are 9/32 x 24TPI. Never seen that one before! And the nuts are 1/2 AF, also a bit unusual for the period.
