Can someone please tell me the difference between a "2b" and "gh3" fit of the thread on a tapped thru hole? i know it has something to do with a roll form thread and this is a very specific protocal?

2b simply indicates an internal thread class, or size range. 3b is a tighter tolerance.

A gh3 is a size specification of the tap's pitch diameter...where a 1 is the smallest and a 5 is the highest, in increments of .0001", depending on the size of the tap.

If you need sloppy threads, you might specify a 2b, and you'd use a gh5 tap. You might also use a gh5 if you're going to plate the part and the effective pitch dia will get reduced.

Now. You can stop asking this question all over the internet! :)

I thought it was increments of .0005
Joe

Note: it's still a range

Example: G H3 indicates a Ground Thread Tap with Pitch Diameter limits .0010 to .0015 over basic

Pitch Diameter limits for Taps to 1" diameter inclusive.
L1 = Basic to Basic minus .0005
H1 = Basic to Basic plus .0005
H2 = Basic plus .0005 to Basic plus .0010
H3 = Basic plus .0010 to Basic plus .0015
H4 = Basic plus .0015 to Basic plus .0020
H5 = Basic plus .0020 to Basic plus .0025
H6 = Basic plus .0025 to Basic plus .0030