SpudFiles

Is it necessary to have a spring to reset the stem? Are there any downsides to not having the spring? My hammer valve has a pretty good seal, even without a spring holding the stem against the seal, I can fill it no problem. When I hit the stem the air pressure inside automatically resets the stem shutting off the valve, should I still use a spring?

The spring is mostly for reseating the valve at low or no pressure. for example when you are trying to charge the reservoir for the first time it has a tendency to not shut, as you don't have enough pressure to seat the valve fully, so all you're essentially doing is blowing air through the launcher.
Also, if you happen to hit hammer valves a bit too hard they have a habit of getting 'stuck' open, and it just dumps your chamber, potentially being quite dangerous.

From a 'functional' point of view they serve very little purpose, in that the launcher would function without one, but from a practical view in the real world, they are quite important.
So it's your call really, a small spring isn't really that hard to put in and can save a lot of hassle.

vng21092 wrote:Is it necessary to have a spring to reset the stem?

Not absolutely necessary, but of particular value with lower pressure(say 1500 psi or less) pcp or co2. Every commercially built valve I've seen has a spring...the .32 pcp pistol I built runs without one, as does a home built Condor I know of.

Are there any downsides to not having the spring?

Ability to tune the valve opening and dwell will suffer with lower pressure pcp or co2. The spring is a significant part of the closing force with these.

My hammer valve has a pretty good seal, even without a spring holding the stem against the seal, I can fill it no problem. When I hit the stem the air pressure inside automatically resets the stem shutting off the valve, should I still use a spring?

Maybe...

If you don't have a chrony to accurately measure results....get to a point where you're happy....and BE happy.