On Tue, 14 Aug 2007 00:12:17 -0600, "SteveB"
wrote:

It makes me curious, though, as to how they would tap a tapered hole, as you
go in, it would become harder until you reach the snap point.
========
Duh moment -- they use a taper tap and only go in so deep.

In the larger sizes they have taper reamers to taper the hole
before threading.

This is common with NPT [t as in taper] series of threads in the
US, and I gather common to the water/gas pipe threads used in
other areas also.

The rationale is that the threads crush at the root and crest as
they are tightened and thus eliminate the helical leak path on
straight threads. (It also helps to put a big glob of white
lead, or now teflon tape on the threads to insure a gas/water
tight joint.)


Unka' George [George McDuffee]
============
Merchants have no country.
The mere spot they stand on
does not constitute so strong an attachment
as that from which they draw their gains.

Thomas Jefferson (1743-1826),
U.S. president. Letter, 17 March 1814.