Hi all,

I recently picked up three large (7") cast iron 3/4" pipe clamps.
Second hand, but never used, and I think I know why now: the two holes
on the threaded side of the clamp don't match up very well.

It's too difficult to even get the thread started for two of them. On
the one that I can actually thread (as long as the clamp bolt is
completely extended), it is impossible to rotate the the clamp bolt
back to the open position.

Anyway, I'm thinking that the only way to salvage these is to bore out
one side of the non-threaded hole until it is parallel with the
threaded hole--about 1/4" by my estimation. The hole is 1" dia., 3"
deep, cast iron.

What's the easiest way to remove the metal? I struggled with a round
file for half hour and barely made headway (maybe 1/64"). I have a die
grinder (but not the proper bit): is that the best way? Any other
suggestions?

Thanks,
H.