SpudFiles

Calculating spring specs and materials just is not making sense to me, and I can't seem to get a clear direction from one place as to how to calculate the specs. first of all is it possible to get a spring strong enough to hold against 500 psi of force before it opens, through a .5" opening on a .5" dia. piston , by using a spring .5" in dia.?

Good luck with that, a spring able to hold back 500 psi will be really really hard to find, it also depends on your chamber volume. The larger it is, the more force the air will exert on the piston.
And If you do manage to find one, I guaruntee it wont be 0.5 in diameter. Plus, why do you need a spring able to hold back 500 psi?

you should just use an air spring, it'll be much easier to figure out

I believe D_Hall sent you a series of links to Wikipedia that should have explained this to you, but I guess I'll do it for you.

Calculating the force on the piston: roughly 98 pounds of force

It is CERTAINLY possible to get a spring that is .5" diameter or less to support that amount of force. Hooke's law: Spring force = distance the spring is compressed * spring constant. Anywhere you buy a spring, the spring constant is usually given. Find yourself a spring that will fit into whatever device you're building, then divide 98 pounds of force by the spring constant and that'll tell you how far the spring will compress to support that load.

For a load of 98 pounds in a .5" ID hole though... I'd go with an air spring. Not only will it be slightly easier to figure out, you won't need a spring in excess of 5" long. McMaster doesn't sell any normal coil springs with a spring constant high enough to support that kind of load in any sensible distance. An air spring will simplify things, but leaks might be an issue.

I'm also interested why you'd want to use a spring to hold something shut at 500 psi, the only application I can think of is a high-mix hybrid.