Mmmmm.... It's old parts of a belt sander I'm remodeling which happen to be imperial . The adjustment bars which are 1/2" have been shortened so I have to recut the 1/2" 24 threads for the four nuts that hold the tension on the belt .The tensioner slides back and forth on the two bars .Tension is held by the nuts one each side .0.058 pitch error over 40mm is more than two pitches total error. No way that is going to work.
What are you trying to chieve and what are you using to achieve it with?
Yeh, going to try on scrap material first .Well, if you're talking jamb nuts then the length of thread is unimportant you may well get away with 1mm pitch.
Bestt way is to try it on some scrap and see idf the nut screws on.
Metric Boxford latheIs it just the nuts you are doing or the thread?
Why not do it the original 1/2"-24?
EDIT
Read again and it is the rods you are cutting the thread on but still why not just do them the same as before?
Are you sure it's 1/2" x 24 is it of american origin as the only thead that would match that is NS ..unless it was one off specific
Tricky to measure is it Whitworth form 55 degree or is it 60