A question about modifying the Discovery valve

[rloftus]

I am building a bottled Discovery with a LW barrel regulated to 1500 psi, and I have done most of the port modifications needed to achieve high 800 velocities with 18g JSB pellets.

One thing really bothers me about the valve on this model.  Unlike the old valves, this one has no o-ring seal around the pin shaft and has a huge clearance between the pin shaft and valve housing.  The pin measures .155 in diameter in a .160 hole, and by area this represents over 6% of the enlarged port area to the barrel (.166 or so). 

If velocity follows pin area loss, 6% of 875 fps is over a 50 fps reduction in pellet speed.

Is my thinking correct in assuming this power loss?   I can correct it by bushing the valve pin hole for a better fit, but I sure don’t want to do it for no good reason.   I really don’t wish to create other problems related to the valve opening properly, and maybe why they left the o-ring seal out of the new models.

Has anyone else dealt with this issue,  have any thoughts about trying to improve the air loss, or just leave it alone

Roger

[rsterne]

I've noticed it, some valves having a poor fit, but never worried about it.... I'm pretty sure they left out the O-ring to save the machining costs of the housing for it.... IF they kept a close watch on the tolerances it wouldn't be an issue, IMO.... but they don't seem to.... A "sliding fit" is about 0.003" and that basically wouldn't leak.... The stem should slide easily with no binding, but shouldn't wiggle sideways in the hole.... If it wiggles around, you are probably losing a tiny amount of air, but not enough to change the velocity as much as you would think.... A more likely culprit is the air leak pressurizing the tube between the front of the hammer and the back of the valve and that air cushion slowing the hammer, and hence lowering the velocity.... My solution has been to grind a notch in the back bottom corner of the valve to allow that air to vent out the 1/4" hole that is behind the bottom screw hole.... In a 22XX it's wide open, but the Disco valve is longer, nearly or completey blocking it.... My Hayabusa had a poor fit on the valve stem and no venting of that area, and a very good (too close) fit on the hammer, and it suffered valve bounce to the degree it fired like a machinegun until the reservoir was empty.... A single 3/16" hole through the side of the tube right behind the valve cured it completely.... and interestingly enough, you can't really even notice the air leak....

Bob

[rloftus]

Thanks for the response Bob.  I never thought of the hammer impact from pin leakage, but I assume there must have been blast of air at just the right time just before valve closing.  In any case, I will open access to the vent hole as you describe by lightly grinding back the bottom of the valve just below the port.

Looking at the QB78 with a similar pin arrangement,  I see about a .0025 clearance on a much smaller 3mm stem.  This calculates to about a 1.8 percent loss which seems much more reasonable.

I have made a tight bushing with about .0015+ clearance for the Discovery valve, but the valve is now sitting in the lathe waiting to be bored out.  I wonder if I will use it or not.  I also wonder if there might be a valve seat alignment problem with such a tight clearance.

Roger

[Ribbonstone]

Have had to do the job a couple of times on old pumpers; often the old ones are shot enough that the stem kind of ovals out the valve head and there is a major leak (along with some alignment problems). Yeah, different power plant, but the valve stem works the same way as a PCP’s.

Hard way is to drill/ream the valve’s head larger, then bush it back down, then drill/ream the bushing to the size of a standard valve stem.  Guess the “classy” way would have been to thread the bushing, but I just made it fit and silver soldered it. Negative is that it’s a lot of work. Positive is that you can use standard valve stems.

Easier way it to ream the valve head to even (for the old guns that tend to get oblong with age) and make a fatter stem.  This can limit air flow (not much of a problem in pumpers) but can get around that by rebating the stem at the air passage. Positive is that it’s easier. Negative is that you cannot use standard valve stems.

Never went to the bother of making an o-ring assembly for one, finding that if it fits “right” the air loss is so minimal it would be a waste of effort.

3rd choice would be (as mentioned above) to vent the tube and just live with it.

Now it has me wondering.  Maybe drilling out the existing port to a way-large size and then adding a Delrin insert  into the valve head to be drilled to be a correct fit to the valve stem might not have been easier.