Bob La Londe wrote:

"Bob La Londe" wrote in message
...
So what's the

spproscribedsp

prescribed 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?

LOL.

Silicone o-ring for modest heat. Upto about 325 degrees.



Most O-rings I look at are seated in square grooves. I believe you need
the extra space afforded by the square groove to allow the O-ring to
deform and seal properly. For a piston though, you should probably be
looking at cup seals and the like on McMaster.com.