wrote in message
oups.com...
I have been pulling my hair out trying to tap holes to get decent
threads. Instead they are always sloppy and oversized.

Only by checking your tapped holes with a proper plug gage can you judge
that the thread is oversized. There must be clearance between the male and
female thread in order for them to go together, and for interchangeability
with standard fasteners. If you're using a bolt to check your threads, it's
guaranteed it will feel sloppy unless it's considerably undersized. Even a
plug gage will have a somewhat sloppy feel if a thread is on top tolerance.
Typically, a no go can go a maximum of only two turns unless it goes all the
way, but very tightly. If it spins in easily, the thread is oversized.

I use good technique.....tap absolutely square to the hole;
I use a good cutting oil; and I back the tap out to clear
the chips; etc.

Tapping is very demanding of lubrication. A good cutting oil may not be
enough. Try to use prepared tapping fluids.

But the fact remains that a 1/4-20 tap measures .255 diameter
while a 1/4-20 bolt measures .244 diameter (threaded portion)

Both perfectly normal. You can not judge a thread my the major or minor
diameter alone. As has already been mentioned, the pitch diameter is the
major feature by which threads are gauged, although all features are
important. As long as the major and minor diameter are not out of limits,
the pitch diameter is what is really critical.


That is .011 difference!!! I have checked my other taps and they are
all oversized.

As they're supposed to be, for the allowance already mentioned.


How can this be and is there any solution?

Yep, use a tapping head if possible, and don't power drive hand taps.
That's an open invitation to breakage. It's also not a very good policy
to try hand tapping with power taps. They usually don't provide the kind
of support needed for helping to keep them oriented at right angles.

Thank you very much

Not sure I helped, but you're certainly welcome.

Harold