I want to tap a counter sink hole through the bottom. Even the bottom tap has
1 to 2 chamfered threads. So the thread will always have 1 to 2 threads 
dead zone at the bottom. Since I need to put a O-ring in the bottom, I need 
a tool that can tap a thread to the bottom without any dead zone (chamfered 
threads). Any suggestions will be highly appreciated. Thanks!

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Clarification of Question by tt1234-ga on 22 Jun 2004 06:09 PDT

Thanks for your help! My question is how to complete thread in a blind
hole. Even if I use a bottoming tap, there is still 1-2 threads gap in
the bottom.
The O-ring doesn't have thread. I need to use a screw to compress it for
sealing purpose. This is why I don't want any dead zone in the bottom.

I wanted to know more about this, and according to this:
http://www.geocities.com/budb3/arts/meth/thrd.html,
Taps come in three styles: Taper or starting, taper chamfered back 10
threads; plug, chamfered back 5 threads and bottoming, chamfered 1
thread. Tapers provide much easier starting and hand alignment. Plugs
are then used to finish incomplete threads. Except for critical work,
plugs are generally used for both steps and are the ones supplied to
hobbyists. However in some shallow blind holes, it is desirable to run
threads close to the bottom for better purchase. A bottoming tap can
easily be made from a plug by carefully grinding 4 threads from the
end and re-chamfering 1 thread. Avoid too much heat, which may soften
and ruin.

This seems to indicate what you are encountering is indicative of the
design of a bottom tap. You already know more about this than do I,
but I'm curious about why the chamfering makes a difference in a hole
in a sink? No, this isn't a "what's chamfering?" question. As I
understand it, chamfering won't make any difference once the tap is
past the thickness of the sink. (if the tap is longer than the
thickness of the sink).

The O-ring, I assume, has threads, and isn't able to complete the
thread through the bottom. Did you also attempt to tap thread both
sides of the hole? I realize that's near impossible unless your able
to measure properly and exactly.

Well, if I'm spouting too much or on the wrong track, I apologize.
Maybe someone else can help.

I'm not a Google Answers Researcher.

What if you cut the screw to less than the unchamfered threads? (I
hope the terminology is correct).

Unfortunate that we can't see a diagram which would make things a lot
clearer. Since the problem seems to be 'How to tap a blind hole to the
very bottom',I would suggest first using the bottoming tap until it
will go no further,then grinding off the one or two tapered threads at
the bottom of the tap and tapping again until it will go no further.
However, the disadvantage would be that you would also grind off the
centrering hole at the end of the tap, making it impossible to sharpen
it, should the need ever arise. Personally I would measure the total
length of the final tapered threads and the thickness of the O-ring,
then fit more than one O-ring if necessary so that their total
thickness > total length of the tapered threads.