Thread engagment rules of thumb vs. published standards
I've searched high and low on the matter of min. length of thread engagement. What I find are rules of thumb but no actual citations that refer to an ASME, ISO, ANSI, or other recognized authority on the matter. The problem is that I'm wanting to tell a customer that there should be more threads in a fitting but I don't want to send them rules of thumb as authortative back up for my claim, I want to send definitive rules or published standards. For example, I can say there should be a minimum of 3 full threads as a rule of thumb, but this isn't stated anywhere that I can find as a definitive design standard. Similarly, a statement like "Basic design rules state that the screw should break before the threads strip." To be clear, I'm not arguing the validity of the rules of thumb, I agree with them. What I'm seeking is a more credible, authoritative, reference that I can use to give a customer. How does one bridge the gap between a generally accepted rule of thumb and a published standard from an internationally recognized institution?