made an adjustable piston for the 880

[Rob M]

Ive been using the last design pictured for a while now , since i posted it.. I was planning on selling a few But i simply could not come up with one simple design that was gaurenteed to do well in every single gun so i opted not to..To test it , i replaced thee compression tube and tried all the same settings, and sometimes it was perfect and others it needed to be pulled and adjusted.I do know that the max of the stock gun in any variation with no increase in valve volume is 910fps with 7.9

[bhh]

I built this today. Results to follow tomorrow.

[Rob M]

that look awesome mman, cannot wait to see results. i like the use of the higher quality orings.

[bhh]

Thanks. The o-rings are Viton 70 in the original metric size. I also have some -113 Viton 90 o-rings that fit a bit tighter that I'll try if these don't hold up.

[Rob M]

yep.. it becomes a tight fit with the dual orings but a really good seal if there were any imperfections in the compression tube. Im actually building a 1 inch bore pumper from scratch right now , and finding more and more that the rigidity of the pump assembly from the handle to to top of the piston really dictates how much energy you can get from it.. your setup looks quite solid so hopefully we will see some good numbers

[bhh]

Im actually building a 1 inch bore pumper from scratch right now

Interesting. Do you have a build thread anywhere? Coincidentally, I found a piece of 1" brass tubing in my dad's scrap pile this week and now the gears are turning...

[Rob M]

no build thread , but you can see the valve i made here


http://www.network54.com/Forum/275684/message/1475020298/so+i+made+my+own+valve++for+a+new+project+%28pics%29

[bhh]

Looks great. Are you planing on a two-stage pump?

[Rob M]

not on this one, this will just be a big bore(.30) sidelever pumper, keeping pump strokes to a minimum. I do have plans for a 2 stage system but not sure what platform yet.Its soo complex the only guy i know to pull it off so far is ron on the green forum circa 2006 i think

[Rob M]

the funny part there was he was able to pump the gun to 6000 psi ! yes 6000 psi lol.the valve split into 2 and the sheer heat above 4500 was melting the check valve

[bhh]

Wow, that's impressive. My goals are more modest. I'd like one pump for plinking (450ish fps in 22) and four or five pumps for 800ish fps for hunting.

[Rob M]

and the biggest challenge there , like it was with the 880 was the swept volume of each stroke.. were the frame not soo restrictive ive had simply added a 1 inch id compression tube on the 880 and instead of needing 23 pumps to hit 910fps , i may have been able to get there in 7-9 pumps.. and modest power at 3-4 pumps.Granted, id have snapped more pump arms than i did lol. Theres soo many pumper possibilities and like you already know , 2 stage is mecca. Is there room in the frame for a bigger comp tube on your new gun , i never looked closely

[Rob M]

heres the plans for 6000psi pumper

http://www.network54.com/Forum/275684/message/1255819561/telescoping+piston+pics

most people wouldnt even believe this is possible with a pumper.

[bhh]

Yes, I think there is room for a bigger tube on the NXG. Now there is another idea swimming around in my head. Lol.

Thanks for the link. The two stage pump uses a ball lock system like is used on the tractor hydraulic couplings I'm familiar with.

[Rob M]

phils design is similar and still in the works.. the only challenge with the bigger comporession tube is obviously sealing it to the back of the gun( the valve area) so if its .730 now and you jump to .860 you will simply need to machine a spacer that seals to those orings, and most likely interference fit to the tube wall itelf for the sake of keeping it compact and air tight.. As the bore goes up the swept volume goes up exponentially..So really even a small gain like .050 is pretty substantial ( too lazy to do math lol

[Underdog]

895fps with the crosman premiers is the max at 20 pumps , theres a thread in the american gate to see the other numbers..Heres the new design , it circumvents the need for a roller pin, and should offer better stabilty to the piston head,. Im not sure if it will increase muzzle velocity yet

Do you have any drawings of this? Maybe I could make my own - even if all I have is a wood turning lathe...

[Rob M]

i think i have one of the old designs in my toolbox.it may need shimming, and needs fatter orings , i dont know what size,but if ya have a kit sitting around it you will have them.. PM me and Ill mail it out, obviously i havent tested this one and cannot recall if it worked as is or how it fit so for FREE i dont expect any negative backlast on the forum ( on PM its fine)

[Rob M]

its one of the them with the drilled shank insert , not the cup design as seen in the picture.

[Underdog]

That's very generous of you Rob! Thank you!
 I'll send you the "flame-letter" on a PM. 

[bhh]

Results are in: 915 fps on 22 pumps.   

Just kidding. Lol   

Real numbers with 7.9 gr CPHP:

3 pumps - 450 fps
10 pumps - 675 fps
15 pumps - 750 fps
25 pumps - 780 fps
30 pumps -  810 fps

I adjusted the piston several times and this was the best. The pump arm was flexing and I had firm contact between piston/valve. I tried the firmer/fatter -113 Viton 90 seals and there was no change, except for the pumping friction, so I think I'm getting a good seal.

I didn't go past 30 pumps because I didn't want to destroy my rifle just yet.  I have a few more things I'd like to try.