thanks for all the input.... yes this thread is a custom "special thread" the we had 2 parts from the 50's but no drawings for them..... So I started measureing the major of the external and using in the machinery's handbook and interpolating the tolerances from a 5", 5&1/8", 5&1/4" came up with the piches and minors of course the thread was about .050" under the what a 5&3/16" would be.... and is the new part were made to 5 3/16 they would not interchange.... so to all thanks for the links to verify I was not out in left field when spec'ing out this thread....