DIN standard set screws that take a 2.5 mm hex key are M5, not M6. McMaster
Carr sells tapered hex keys for removing socket head screws that are
stripped. A cheap trick is to find a torx bit the right size and drive it
in (we use T20's to extract shcs's that take a 3 mm hex key, works most of
the time). Heat is really your friend, especially if Loctite is involved.
If you don't want an open flame use a small heat gun and get it hot enough
to make water almost sizzle. Maybe let it cool completely and reheat two or
three times before trying to turn it.

-----
Regards,
Carl Ijames

"Usual Suspect" wrote in message
obal.net...
Deep in a large machine (offset printing press) an allen set screw holds a
shaft in a casting. The set screw is also locked in place with a lock nut
(the screw protrudes from the casting enough to fit the nut).

The allen wrench is 2.5mm and the lock nut is 10mm so the diameter of the
screw is probably 6mm (1/4 inch)...

I removed the nut but the allen wrench (a new one) is slipping in the
screw.
How do I go about removing this? Access is only head-on (no side room for
vice-grips or such). The environment is one where lubricating oil is
literally raining on these parts so the "blue flame wrench" solution is
out.

I tried a bit of banging on the side of the screw (yes, funged up the
threads
so the nut won't go back on) but this didn't budge it.

Are these allen screws hardened? Is it OK to try drilling for a screw
extractor?

Thanks,
--
Al, the usual