No, I think it's the over-the-thread diameter for the taps that's needed - for the threads to run together a female thread has to have clearance at root and crest, so the crests and roots on the tap have to be slightly over nominal size.
Machinery's Handbook gives 1/4" NC as .2532 - .2557", NF as .2524-.2549" etc. By the time you get to 1"-8 the oversize is between 8 and 12 thou", a tap will definitely not go through an accurate 1" hole without scratch-cutting threads! Or a hammer and drift ;o)
Dl, you may be able to lap the tapping block enough to get the taps though, use an on-size rod and abrasive in the drill and keep feeding it more abrasive until you hit the max. tap diameter from Machinery's - it'll be a slow process though. Or... next size up in drill:
1/4" close fit F, free fit H;
5/16" close P Free Q;
3/8" close W free X;
7/16" close 29/64", free 15/32";
1/2" close 33/64", free 17/32".
For a tapping block you want close fits to keep the tap square to the work face, I think...
All this info' is in Machinery's, it's pretty much an essential in the workshop (though I keep mine indoors away from the dirt and grease and take notes to the bench), I really recommend buying a copy! I have the 13th (1948) edition in paper form and a recent electronic copy - I find the paper one has all I need and it's a lot easier to find things than the electronic!
Dave H. (the other one)
