I've got to cut a 5BA thread on something which is a horrendously inconvenient 43 TPI. Can anyone suggest a set of change wheels running off an 8 TPI lead screw (Myford Super 7). A tiny error might not matter but I need to be close. While my maths is good and the principles understood, this is taxing me. Comparing notes will help.

Thanks everyone for the replies. I managed to get the thread calculator working and managed to get it within 1/10th of a tooth over 1" which is more than enough as I'm only machining about 3/8th" of thread. In answer to the question what's it for, I'm machining the body threads on an old Devilbiss airbrush and although the bore is about 11mm, the thread is the same as a 5BA machine screw. I suspect they may have used 5BA Coventry die head chasers as a cutting tool. It's not 44tpi and it's not 42tpi. The only thread I can find inbetween is 5BA which is 43tpi. I may braze up a tool from a die head as a cutting tool. The important thing is it's close enough to do the job.

I know Everything Airbrush as they are the official inheritors of the Devilbiss brand and spares. One of the guys there worked for Devilbiss assembling the airbrushes on the production line. It's not 0.6mm. I took a silicone mould of the internal thread and then compared it to various thread gauges viewed through a jewellers loupe with an engraved scale. As mentioned, 44 gauge is too fine and the 42 gauge is too coarse. Incidentally the originator of the DeVilbiss Aerograph airbrush was American but they have been manufactured exclusively in England for more than a hundred years, essentially unchanged apart from modern materials being introduced. The external air valve thread as mentioned is 3/8 UNEF and the thread in the handle (still available) is 5/16 x 32. The thread I'm chasing (see what I did there?) is the thread in the forward part of the body. All the dimension of the airbrush appear to be imperial (needles are 3/64" diameter rather than the more common 1.2mm. Unless anyone can suggest another thread with a virtually identical pitch, 5BA (43 tpi) is all I've got.

That's a thought. The thread is so fine though I'm not sure the actual thread form is going to play a big part. I have to say looking at the thread, it's more akin to a regular 55 or 60 degree thread form than the rather more rounded BA thread.