The "square" rule applies to bolted joints where the strength of the material of the internal threads (a nut or tapped hole) is the same as that of the cap screw or stud. The general rule for a high-strength steel stud threaded into cast iron is for the length of the good tapped threads to be 1.5 times the diameter of the stud or cap screw. Thus, a 0.500" diameter stud should be installed in a tapped hole with 0.750" of good thread.
Hoyt, II#922
