[Shop-talk] retapping a hole

[Doug Braun]

No!  A 1/4-20 thread has a smaller minor diameter, so there is not enough
material.  Definitely use a 1/4-28 heli-coil.
Fortunately these days, there are other manufacturers selling "thread
repair kits", so it will be less expensive than you fear
(just check Amazon).

You may need to drill the hole a bit deeper (with the drill meant for the
heli-coil) so that the tap can reach in far enough.
A side benefit is that the  heli-coiled thread in pot metal will be
stronger than before.

Also, a 1/4-20 coarse-thread screw would come loose more easily  than a
fine-thread one.

Doug


On Sun, Jan 15, 2023 at 10:54 AM john <jniolon at att.net> wrote:

> I have a mirror mount on my truck door that held a rear view mirror.
> Unfortunately the mirror decided it didn't want to stay with my truck.
> So, I'm looking for a replacement. The mount is drilled and tapped for
> 1/4"-28 threads and most of what I find is 1/4-20.  I'm wondering if I can
> re-tap the metal over existing threads with a 1/4-20 thread and it will
> hold...or will I just make a mess of it and make it broke. The drill side
> for the
> two thread sizes are within a 64th or so of being the same and the mount
> is 7/16" or so wide so I have some meat to play with...
>
> suggestions?
>
>
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