Thanks Kim. I was fairly happy with it, except for that darn leak. Thought I could hold out until after the FT meeting on Sunday, but this morning the darn thing was at zero pressure when I went to top up. Leak obviously getting worse. So I got a few proper jobs done, fixing some fiddles, then took the 2240 apart again. What I found was a destroyed delrin seal:
Looked to be about 0.02mm of delrin left, and that was fairly bubbly and translucent. Not exactly what I was expecting. I thought maybe the delrin was just too slippery and air was slipping past it... but no, apparently 3,000psi boiled the plastic, or at least pushed it into a liquid-like state such that most of the seal flowed out into the recess in the brass of the valve. Some of it went right down into the valve as well, closing the port partially. I couldn't chip it out with a knife, had to chuck it in the lathe and cut it away from the brass as it was thoroughly bonded. Yup, definitely melted in under pressure.
So I went to my toughest plastic rod, Ertalyte. This stuff is very stiff, has a much higher melting point than delrin, and is designed to be extremely resistant to deformation under considerable pressure. I hadn't used it previously because I didn't think it'd deform enough to form a good seal, but the delrin experiment proved I was under-estimating just how much damage all that pressure can do. So I turned the seal port a bit deeper, maybe 1mm, and opened it out slightly to be rid of the dovetail, actually making a slight step about mid-way, then cutting off the face of the lips to maybe a couple of millimetres lower. Barely kissed the bleed hole on the one edge of the Crosman valve seal seat. Then I made an Ertalyte seal to fit the new opening. Hole matching the port in the brass. Press-fit into the valve. Face contoured to be a snug fit on the nose of the HiPAC.
I assembled the gun and threaded in the HiPAC and pumped it up to 1,000psi. Listened, and no leaks, so I pumped to 3,000psi. Still nothing. Absolute quiet. So it was Chrony time! Oh yes, I was mistaken earlier in saying I'd used a 25mm spring; it was a 30mm long chopped spring I used, the '3K' one from Powermax.
What I found was that turning down RVA preload more frequently eliminated at least some of the spikes and valleys in velocity. So I'll probably make a smaller pin tool, something I can just leave in place while looking through the scope (the current tool is too tall when in the upright position in the RVA bolt) and adjust every couple or few shots, backing off maybe 1/8th turn or less each time. I added a nut to the RVA bolt, loc-tited it into place then turned it down in thickness until it was giving my average velocity from the first shot at 3,000psi when fully dialed in. So my starting point is my average velocity at full pressure, then I just back it off every few shots, finally getting to about 1.8 turns out from maximum for the last series of about 10 shots.
My new total with the HiPAC leak fixed is 55 shots, finishing at 800psi left in the tank. WOW! I had sort of given up on that high a shot count. When ordering the HiPAC and 1 extension I'd hoped for maybe 40 shots, or a few more, but since running from 3,000psi made for such a huge gap in velocities through the fill it made no sense to pump that high; I was settling on pumping to just 2,200psi and getting 20 or 25 shots. More than double that now, and only the added fuss of keeping track of how far to back out the RVA, that's just excellent. Took a lot of custom stuff to get here. A home made delrin poppet seal, lots of tweaking in the valve volume and port, custom transfer port, hammer mass dramatically reduced, spring length and RVA endlessly tinkered with. Far too much work to stop the HiPAC from leaking. But it's done.
Now I just have to learn the new routine, find a method of remembering where I'm at with shot counts and RVA adjustments such that the process comes close to automated. I'm thinking of making a pellet holder out of hard foam with 55 holes in it, pellets arranged in rows with gaps to remind me when it's time to drop spring tension as I load. Mount that on the side of my neoprene sleeve maybe.