[Shop-talk] What the heck screw do I need? And where do Iget one?

[Randall]

> I have a number of door knobs that need set screws to hold them to
> their spindles.  (Square threaded spindles in mortise locksets.)  The
> normal size of these is 1/4-20 or rarely 5/16-24 or even more rarely
> 1/4-28.  The existing screws are 28 tpi, with a major diameter of
> 0.280.  

Is there any chance they are 26 tpi instead of 28?  The closest thing in my
thread chart is 9/32 BSF, which has a major diameter of .2812" and is 26
tpi.
M7-1 would be next closest, .2756" and 25.4 tpi.

It's also possible that they are simply not a standard screw thread at all.
I wouldn't think you would find non-standard threads as recent as 1925, but
odder things have happened.

One potential solution might be to put 1/4-28 Helicoils in the holes (and
use 1/4-28 set screws).  The fit wouldn't be perfect (Helicoil tap is
.2765"), but I think would be close enough for a door knob (maybe with a
drop of Loctite to be sure).

-- Randall