Jim Reed wrote:
I just got the first 3 jaw chuck for my lathe. I was using a 4 jaw for the
last two years. It just got tiring trying to true up the work and not mar
it in the process. I never did get too good at it.
I put the 3 jaw on the lathe, checked the body for runout, and it did not
register any error on the dial indicator. Chucked up a 1.75" dia. piece of
steel, and made a light cut on it. It measured .001 runout after the cut.
Probably due to rough surface finish.
Then turned this piece around and chucked it in the 3 jaw, using the cut
surface as the holding location. When I measured the runout (still using
the cut surface to measure), I was indicating .020 out. I tried this
several times and it varied from .005 to .020. I found some rust on the
inside of the jaws I thought might be causing the problem, but removing it
didn't seem to help.
My lathe is a Reed-Prentice 16X54, the chuck a Skinner 8.5" 3 jaw I just
pruchased on Ebay. The chuck has an integral L01 spindle mount, and it
looks like it has sat and gotten a little rusty for most of its life.
Any constructive suggestions would be appreciated.

I'd start with disassembly and chemical rust removal.