galling

Once again I'd like to draw on the collected knowledge of this group:

I need to fit a short cylinder (1.5" diameter x .4" long) into a hole. The
fit will be very tight- I may lap them to get the .0001" or .0002" fit
necessary. No lubricants allowed, since this will go into a high vacuum and
needs to be clean.
The fit should be free, as it has to be able to move slightly.
The hole will be in a piece of stainless steel, 303 has been suggested.
My problem is this: galling.
Will the short cylinder gall as it is put into the hole?
Should I make it out of brass, and if so, what kind?
Any ideas about dimensional stability for these materials?

They will be annealed after rough machining. Evidently 303 has less
tendency to work harden than many other steels, it can be used for thread
rolling.

And yes, there will be a computer controlled motor involved, making very
tiny motions.

Thanks for the thoughts.

Elliot