Good info guys. I forgot to mention a cheap bolt sizer could help some folks. Click
Alvin was the first to mention

These two thread checkers are sort of what I was talking about, just get some bolts and make your own "go/no-go" set, as I called it.
Click
Click

Here is the thread gauge I use that came with tap and die set. Click

SAE threads (Society of Automotive Engineers) is standard American threads. UN Unified National. Sometimes called "inch". As opposed to Metric threads. The Automotive industry actually started the whole Engineering world to evolve and begin to standardize.... today we have ANSI specs and etc..
Also a shout out to Andrew Carnegie for his early Engineering and Technical schools where my Dad went in the 1920's/30's.
Henry Ford for his assembly line process and his use of tolerances, to be able to use any part in the bin to fit any part in another bin, is an overlooked massive advancement. He was the champion of interchangeable parts back then. I am the champion of TMI.

Phil,
Machinists: When I worked, I made sure I was good friends with the machine shop. I learned a lot from them about how to design for manufacturing needs and the tolerances they and their machines could hold. Those guys did not get much credit except from guys like me. They found my errors.
Once we got it down, a part was released to a vendor for quantity production.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn