[Top] [All Lists]

Re: [TowerTalk] pipe tap drill size

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] pipe tap drill size
From: Charlie Gallo <>
Reply-to: Charlie Gallo <>, "Tower and HF antenna construction topics." <>
Date: Fri, 12 Mar 2010 08:13:35 -0500
List-post: <">>
On 3/12/2010 jimlux wrote:

> And sometimes, it's stamped on the shank of the tap.

OK - The answer given is accurate enough, as it is (given the fact that you had 
to ask the question)

That said, the answer isn't as clear cut as #7 drill for 1/4-20

First - there are basically 2 types of tap for our purpose here - "cutting" 
taps (which are the taps you will see about 99% of the time - be they 2 flute, 
4 flute, hand, spiral point, whatever) and Forming taps (which don't remove 
metal, but push the metal from the bottoms of the thread to form the crests - 
they only work on ductile materials, and the threads look weird, like they have 
a groove at the TOP of the thread)

Anyway, sticking with "cutting" taps, a #7 drill will give you a 70% thread, so 
it's actually a fairly good answer, but there are cases where you want a HIGHER 
percentage of thread (you never go to 100) or lower percentage (easier to tap, 
somewhat weaker threads)

IF you don't have a #7 drill around, you COULD, in theory go as LARGE as a #4 
drill bit, and only get 57% of thread (I wouldn't ever GO that low except for 
threads that don't really hold anything), and go as small as a #9 drill 
bit(which will give you 77% of thread).  The danger there is breaking taps, and 
if you have a slightly large screw/bolt it won't go in real easy)

IN that range is a 13/64 inch drill bit, which will give you a slightly looser 
thread (66%) than the #7, BUT might be a LOT easier for you to come by
73 de KG2V

For the Children - RKBA!

My Website:
My Blog:

A rose by any other name would be "deadly thorn-bearing assault vegetation."


TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>