Hi there,

The explanation for what you have found depends a bit upon which thread form is involved.

If, say, your tap is to cut metric thread form, the male screw OD is the nominal OD but the female thread (i.e. the nut) has clearance to ensure it clears the crests of the male screw. In order to cut this clearance, the tap diameter has to exceed, by a small amount (* see below) the nominal OD of the thread.

Before posting this, I consulted Tubal Cain's 'Model Engineers Handbook', pages 57 & 58. He shows the profiles for both nut and bolt for several thread forms. His diagrams show the matter more clearly for metric threads than for, say, Whitworth form but there has to be crest clearance built in to the standard somewhere.

What you have found often goes un-noticed because of the difficulty of measuring the OD of a three-flute tap whereas your tap is four-flute where the OD is more easily measured.

I had the same experience when I made a drilling/tapping jig to put the 1/4" BSF screw-holes in the side of my ML7 bed to accept the fixing screws for the quick-change gear-box. I carefully drilled the holes in the tapping jig 1/4" only to find I had to open them up to let the tap enter.

I recommend Tubal Cain's book but I'm sure there are lots of charts of screw-thread profiles on the web.

I hope this helps.

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 24/05/2016 21:16:54

There is another reason for unusual size taps, Helicoil ones. Been there, done that! :>(