"Bob La Londe" wrote in message
...
So what's the proscribed method? I expect I need a pretty consistent
radius bottom. What's the best way. Make a rough semi V lathe bit, and
then make a decent round one to finish?

It has been a few years, but like others have said, just go with a square
groove. Seems to me we would make the groove same width as the o-ring, and
the depth 90% of the thickness, .250 thick ring, cut the groove .225 deep.
Now that depends on your clearance from the piston and cylinder wall too, so
I suppose you should go with the cylinder diameter, minus 2x of 90% of the
o-ring thickness. so .250x90%x2 subtracted from the cylinder diameter.