Usual Suspect wrote:

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'd look at what imperial size fits and try driving that in. On cnc's, I'll use the
turret to hold my breaker bar tight into the screw using the chuck to hold it in from
camming out.

Is your allen camming out?

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller