I will buy you a new motor mount and give you $100 if it hurts your threads. I will give you a $1000 if they were actually made to 3A. The Aerospace industry I was in for 40 years used 2A and 2B. As pointed out, it's .10 of a frog hair. Put the money in the dog shelter. They probably couldn't hold that tolerance in 1945 in a Fastener shop in 1 out of 10. A looser tolerance bolt will fit a tighter tolerance tapped hole. (if the hole is even 3A to begin with) Thread designations have evolved a little as pointed out, and as evidenced by the fact that they did not use "A" after the 3 to designate male. BTW: there is no such thing in the general machining world as 0 tolerance. Especially in 1945. Everything has a tolerance or there would be untold waste at the inspection table. Non-interchangeability of parts would make Henry Ford roll over in his grave. The machines have tolerance, the humidity & altitude will play a roll. Some fine EDM, chem etch type metal removal can produce good, tight, cosmic, nuclear, micro, plasma, Zombie tolerances. But not screw threads.
To say that the 2A is tighter tolerance than the 3A is a misnomer. They have the same tolerance +/- .00435. The 3A is spec'ed to the exact screw size (9/16).5625 and the 2A is not. The 2A bolt is not as large. The correct terminology is the 3A has less allowance.
