Posted this on another forum, but it might be helpful here.
I don’t like “extra†parts. QB tanker with a standard valve (like the QB79 or a tanker conversion of a QB78) still has that piercing pin, which does nothing good for gas flow. The pin adds a bit of random friction as sometimes it centers the hole in the valve for no friction, and sometimes it’s off center and rubs the sides.
It also blocks gas flow, which may or may not be important. Blocks it by being physically there and by requiring that small sized gas entrance (to help center the pin to the 12gr. cartridge seal).
To find out how well a rifle would do without that piercing pin, decided to cut up a beat up, blown seal, spare valve. Would attempt to use as much of the original valve as possible.
Standard valve:
First to go were the little screen and fiber filter at the far left. They do help center the piercing pin, but they both tend to wear and shed fibers which often get caught in the valve seal. The piercing pin needed go go as well, but without it there is no place to anchor the valve stem spring.
There is a retaining tube (black in the photo) that sits on a washer. That washer fits on a ledge inside the valve. By cutting the retaining tube in half and dropping one half in the valve first, the tube rests on that ledge. Which will let you put the solid washer on top and that washer gives the valve stem spring a place to tension itself.
Latheing a new Delrin face to the valve, also shaped the brass end of the valve to retain the spring (also lightens the valve stem a good bit which may have something to do with the results). Spring slips onto the end of the valve stem now, like a short spring guide.
The solid washer was slotted around it’s edge to allow better gas flow and the intake end, no longer having to center the piercing pin, was bored out larger. (The lathe work around the rear end is for a different experiment).
Really tried to get the spring tension to feel the same between the standard valve and the modified one. Can adjust it by cutting the valve stem head back a bit more, by shortening the tube, or by grinding on the spring. OF the three, decided that working on the spacer tube was the easier. that “cutting it in half†is just an estimate...by filing the ends a bit and a lot of testing, can get the spring tension about the same as a standard valve.
Test rifle was a QB78 (my oldest one, and the one that always seems to get picked out for testing) that got rebarreled to 5mm. Did the standard small mods, bolt probe...poly port...debrring and smoothing. Got converted to 850psi HPA along the way.
Results:
With the standard valve and the mods mentioned, running 850psi, rifle was shooting :
Benjamin Cylindrical : 611fps/ 11.8foot pounds
Eun Jin 23.6gr. 501fps/13.1 foot pounds
With just the valve changed to the one show above:
Benjamin Cylindrical: 664fps/14 foot pounds
Eun Jin 23.6gr.: 535fps/15 foot pounds
Conclusions:
There is plenty of room for increasing vel. Adding just a small washer to the hammer spring brought the speed up to 678fps with the 14.3gr. pellet. With a few other changes, line a bit more hammer spring tension and a bit less valve spring tension, am sure it could shoot 14.3gr. a bit above 720fps/ for about 16 1/2 foot pounds if that was the goal.
It wasn’t the goal here. Wanted to improve gas flow but also hold the vel. down to about 650fps. I’m a little too fast for what I wanted and will detune it a little bit to get back down to the speed I’d prefer.
Couldn’t make the before and after exactly the same, but the valve spring was traveling the same distance, was using the same spring, and even with my eyes closed it felt the same as I manipulated it.
The valve stem weight became lighter, and the weight of the piercing pin was removed, so the hammer had to move less mass as it struck the valve stem and that should be worth something.
IF i ever fell like doing this again, will NOT modify the valve stem’s weight...but will still remove the piercing pin as that’s kind of the whole point.
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Ddi the detune by removing some weight from the hammer and have the rifle shooting 14.3’s at 645fps (which is right at my goal of 650).
Will test a full tank tomorrow, was running out of day light today. Found that from 2000psi to 1000psi, was getting a full 48 consistent vel. shots. Should be close to 90-100 shots per 13 ci bottle fill when running from 3,000 to 1,000psi.
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While i did use a mini-lathe to do the work, it could be done with an electric drill, Dremel tool, and some neelde files. Would need a lot more time and care, but so what?
