SpudFiles

I'm not sure if anybody has posted this yet, but if you type in "piston cups" into McMaster's search box, there are some pretty useful items that come up.
There are different sizes of piston cups, but here is the part number to a 4 in diameter one - 9411K28

That's an interesting find, I find this very useful and thank you more than you know.

thanks this is a great find Ill order one soon and test is out.

Nice find, should work great for piston valves, especially coaxials. I've been working on a project that might need something like this. Thanks for posting.

Here is a link for lazy people
http://www.mcmaster.com/nav/enter.asp?partnum=9411K28

I think you'll find they work rather well.