Hi ARM,
I am not sure where you are going here. The PUBLISHED sizes of 5C collet outside threads that I could find while searching some time ago are from 1.238"-20TPI to 1.240-20. Published sizes for the internal thread are from 1.041-24 to 1.047-24. So there is some disagreement among those posting the data. There is for sure some size disagreement in what collet manufacturers actually ship, which is all that really matters. Whatever size YOURS actually are, you need to make them fit. If you have different brands around, measure the actual thread dimension of the collet you want to thread too, outside threads for most drawbars and collet closers, inside dimension for most collet work stops. In the case of a 1.240-20 outside thread, the thread depth you would like will be about .050" for a 75% thread, so sneak up on a root diameter of about 1.140", testing the fit as you get close. When you are final cutting the threads, your collets becomes your test gages. If you have multiple brands and lots, try them all and see if they will fit when you get close to the finished thread diameter. You will find that some will fit tighter than others. Open it up until all will fit or until you are concerned that the smaller ones are getting looser than you would like, at which point you can get rid of any oversized collets and get something else, or make it fit the bigger ones and get rid of any loose fitting ones. They will probably all work OK at some unmeasured but trial tested size. The 5C internal threads are not designed for use with a drawbar or collet closer. Make your work stop fit those threads. Am I answering your question?