Norm Dresner wrote:
I confess that it's been 50 years since I took shop classes in college and
while I've done some things regularly since then both professionally and as
part of my hobby work, I've had very limited experience in certain areas.
SO ...

I've got a bar of aluminum that I had to drill and tap several 1/4-20 holes
in. The bar is roughly 7"x1"x1.25" and drilling and tapping the holes
through the thickness (1") was a good refresher course, but the hole through
the height (1.25") has me stumped. Without thinking it through, I drilled
the hole and started tapping. But the tap's cutting length is only 1" and
the shaft behind it is too thick to pass though previously tapped metal.

A Google search reveals that there are some custom taps that are made for
extra long holes but for a one-off I don't need a custom tap. I think I
found that there are some types where the shaft is narrow enough to pass
through the previously tapped hole to allow deeper holes to be done, but the
nomenclature is confusing.

I need help and I'd love to learn the right terminology. As a start, to get
the project back on track, I'd really appreciate it if someone could tell
me, perhaps, the name and part number at ENCO for the right tap for my hole.
As a follow-up, perhaps a link or three to pages which explain the
differences would also be greatly appreciated.

TIA
Norm

There are several kinds of taps with extra-long shanks, called pulley
taps (from their use in tapping pulley hubs), extension taps, or long
reach taps.

The words you're interested in are "reduced shank" or "undercut shank".
An ordinary 1/4-20 pulley tap will have a 1/4" diameter shank, while a
reduced shank tap will have a shank smaller than the root diameter of
the thread. So you can keep going and going ...

Spiral point (gun) taps are particularly nice here, since they throw
the chips ahead of the tap and they don't clog the flutes.

In your case, see http://www.use-enco.com/CGI/INSRIT?PMAKA=325-4824
for #325-4824 -- a 6" long reduced shank 1/4-20 spiral point tap for
$20.95.

I've only used mine a few times, but I'm very glad I have it.

Tove