View Full Version : 3/8"-16 Tap Problems

07-30-2015, 01:30 AM
My grandfather left me a lot of his tools. In them was a wood box full of different taps. I took out a 3/8-16 the other day and used it. But it wouldn't accept the bolt. I took out one of my Craftsman taps and ran it in the hole and the bolt when in smooth. I could feel the Craftsman tap finishing the cutting of the threads. I double checked the older tap and it's stamped 3/8"-16.

How could this be? Could it be a under sized tap. Maybe to hold the bolt better, like a lock nut? But I couldn't even start the bolt in the hole.

My grandfather repaired the machines in the shop as well as run them. Could this be a special tap?

Hopefully someone can figure this out.

Thanks, Tom

07-30-2015, 02:34 AM
If you look at the shank of the tap,you might see something like an H or G with a number from 1-8 behind it.Look and see if there is anything like that and if so tell us what you find.

07-30-2015, 03:06 AM
My grandfather left me a lot of his tools.How far back would the 3/8"-16 tap date to?

The United States Standard (aka American Standard) thread held sway from the 1860's until
the late 1940's when the Unified Thread Standard UNC (Unified National Course) was agreed
upon. Apparently, while interchangeable, there are differences in tolerances which perhaps
explains what you describe.


07-30-2015, 04:13 AM
could it just be really dull?

07-30-2015, 06:13 AM
It could be 3/8" Whitworth.

07-30-2015, 12:02 PM
I may be the first or second in a 3-tap set of series taps, rather than the third "finish" one.

Series taps sets aren't as common here in the US as they are in Europe, but do exist. Most would have ring grooves or other markings to differentiate from regular taps and each other.

07-30-2015, 03:11 PM
I thought the OP may be referring to a die, and not a tap, saying it would not accept the bolt and might be undersize. Some dies have a greater taper and are easier nto start, but it seems unusual that it would not accept a previously threaded bolt.

But then it seems that the tap was used to make a hole which then would not accept the bolt, so it is most likely a tap. It's probably dull, or perhaps modified by grinding the cutting edges down in an attempt to sharpen it, or to make an extra-tight fit or a locking tapered thread. Some pictures would help immensely.

07-30-2015, 08:29 PM
I also have a bunch of taps and dies my father used in his career as a heavy equipment mechanic. He would
buy new taps, for example, for making new holes. But when they became too dull to use for new work the
tap or die was relegated to the "re-thread' box. These taps and dies were used to clean rust and junk from
threads before re-assembly. A a result most of them are worn enough that if one were used to cut a new hole,
if you could, the thread OD would be too small and need to be run over with a 'good' tap or die to clean it out
enough to fit a new bolt or nut.

I myself use the same stingy system. I won't use my 'good' taps to clean junk, especially corrosion, out of threads.
And some of my taps, maybe only 25 years old, are worn enough to cause this. Been there.....


08-05-2015, 02:01 AM
Thanks for all the replys. I just got home from vacation and I checked the tap. Here's what's etched on the tap-

S. T. Co.

Does that help to decide if it's a full sized tap or not?

Thanks again

08-05-2015, 02:46 AM
G = ground finish on tap
H3 = tolerance designation (+0.0010 / +0.0015)
NC = National thread form, Coarse pitch series
HS = High Speed Steel (material tap is made of)

08-05-2015, 10:02 AM
Taps are throw away tooling.
When it gets chipped or dull,
throw it away and buy a new one.
Are you emotionally attached to
the tap or something? It is not
a purple heart medal.