Class of fit is between the two components. Essentially class 1 is a loose
fit.
Class2 is a free runnng fit. Class 3 is light interference fit. Class 4 is
a wrench fit.

If specifying a tapped hole, it is usually like .... 1/4-20 2B
The threaded component would be... 1/4-20 2A

Looking at apppropriate references (Machinery Handbook) gives specs for
these
classes of internal/external threads.

Then you select necessary tools to achieve these various spec's.
For example, you may use a "F" drill as a tap drill, but if it drills
oversize, you may have to select a smaller drill so you end up with
the correct sized hole prior to tapping.

The specs for bolt strength is an entirely different subject.

Good luck!










wrote in message
oups.com...
Thanks guys for all your help. I still have some more questions.
First of all to answer Pete's question, the fit of the bolt is really
loose, especially the first 3 or 4 threads. If put to the test
they usually will strip out.
Question: Does the class of fit (1 thru 4) refer to the tap or the
bolt?
In all of my catalogs and all my internet searches I have never seen
a class 4 bolt. Usually they are referring to the hardness of the
bolt.
In any event, does anyone know of a source for class 4 bolts? Also
a source for quality taps?
Finally, what do you guys think of a spiral pointed tap for open-ended
holes?
Thanks so much.