The tolerance is much tighter for the 3B (female treads) than for 2A (male threads). The slop will damage the mount parts.
3A is a much higher tolerance... they are typically not die cut, but cut on a lathe and not for mass production.
The NF-3 standard still applies today as you can get NF-3 spec bolts... although typically as cap head screws.
Don't tell the aviation industry that they are using outdated specs.... the AN system, for one, is also alive and well.
Last edited by pfarber; 10/23/2013 2:26 AM.
