What is the rule of thumb on thread engagement in through holes?
I only ask because my searching keeps coming up with 1.5 x Dia, so an M5 would require 7.5mm of thread.
This rule would make it impossible to construct a steel frame from SHS and drill, tap and bolt into it.
If my memory recalls correctly I thought it was a min number of threads, approximately 3 full threads or something like that. Please correct me.

Jim