made an adjustable piston for the 880

[Rob M]

aside from the transfer port, what else do you think is a limiting factor? I do think a heavier hammer spring would gain you another 20-40 fps if you happen to have a thicker one laying around.. does your machined piston eliminate head sPACE?? i know on the 880 having the piston literally crashing into the valve check block at the very end of the stroke created the best numbers.. ( i do mean barely , it was probably impacting .030 before the lever closed)

[bhh]

I just took it apart and found a thick layer of blackened silicone oil on the valve and piston face. I think the oil was keeping the piston and valve from touching and causing the hole in the lever to stretch until it broke. I put the lever pieces back together and it looks like the hole elongated before it broke. Maybe that accounted for my decreasing velocity?  On the good side, none of that blacken oil had migrated past the o-rings so I believe I was getting a good seal. 

During shooting today I checked and the valve was not retaining air. Would a stiffer spring do anything in that case?

So, build a new arm, reduce the amount of oil, and make sure the valve and piston touch. Then try again with about half of my additional volume.

I had the piston and valve face touching when I installed the new piston. I can adjust headspace slightly by loosening the valve half.

[Rob M]

the elongagting of the hole in the toggle created headspace over time, as did the accumulation of grease.. So thats a big reason you stopped building pressure at some point ( pump30 ish ) As for  The hammer spring. people call it lock time but yes.. As the pressure increases, the hammer spring may still be able to blow open the valve, But is it doing it quick enough to make use of the new found volume?? So you may be reaching this fine balance point where the pressure is low enough to let the valve blow open( barely ) , but its not doing it quickly enough because of the( well beyond stock)  back pressure. If that makes any sense?

[Rob M]

for example , the 880 trigger is brutal at about 8 lbs , but a good amount of that sprung weight is the hammer spring.. So it had no issue at all dumping 2200 psi.. Could i have made gains with an even heavier spring, perhaps , guys with far less mods gained 20-30 fps by making the hammer violent.. I will say this , ive moved on from the 880, but when someone comes along and slings a 7.9 from an 880 at 911 fps , ill be right back at it with a 2 stage piston! . I simply dont see that happening  ( thats not cockiness, I hope someone does make a 1000fps 880

[Rob M]

in other news, the p17 set up for 3 -4 pump max is brutal, on the 4 th pump , the effort is about 100 lbs.. my hands are destroyed. The bad part is its impossible to take the tube off and add some more valve volume, so each revision requires a new gun.. on 3 pumps i got a lame 480fps with 7.9 grain..if the roll pin in the piston would stay in place for pump 4, im pretty sure i could crack 600 fps( blood adds 20 fps)My new idea, if i decide to pursue it , is a true 10 pump msp gun.. the idea being that the gun would not need to exceed stock pressures by much , but the volume would be triple.Thats the only confuration the gun would have mass appeal , and a key reason i believe a prominent builder has taken soo long to get the system to market.A very bright and sneaky way around this entire challenge would be to make a sleeve reducing the piston bore to .700 from .985..the sleeve at the rear could also be the check valve, This way the pumping effort to accomplish 3000psi weould be substantially reduced . Lets look at piston face pressures to get to 3000psi .. with the stock piston , its impossible. ( .4925x.4925X pi X 3000psi =2284lbs /15:MA =152lbs effort for one pump , NOT HAPPENING, the gun breaks at 100 lbs , i KNOW
for .700 the face piston pressure would be .350X.350XpiX3000=1154lbs at piston , given a  mechanical advantage of 15:1 reduction , were at 77lbs effort. Very doable since i know i was pushing 100 lbs effort today.. The downfall to this design is that even with double valve volume , (ill have to do actual compression model math ) you would have to pump about 20 times to reach the peak pressure. Whereas with the full size stock piston , 8 pumps would give pretty dam good speed with normal effort. All a balance, as usual

[bhh]

the elongagting of the hole in the toggle created headspace over time, as did the accumulation of grease.. So thats a big reason you stopped building pressure at some point ( pump30 ish ) As for  The hammer spring. people call it lock time but yes.. As the pressure increases, the hammer spring may still be able to blow open the valve, But is it doing it quick enough to make use of the new found volume?? So you may be reaching this fine balance point where the pressure is low enough to let the valve blow open( barely ) , but its not doing it quickly enough because of the( well beyond stock)  back pressure. If that makes any sense?

Yea, that makes sense. A quicker spurt rather than a longer squeeze of air. I'll poke around and see if I can find a stronger spring.

So, sounds like I need to get an 880 and see if I can get 911 fps....Lol

Bummer about the P17, but you're right about balance. As I was pumping 40-60 times for each shot I realized that I'd rather have 750fps at 12 pumps than 900 at 40.  But it's been a great learning exercise.

[Rob M]

yessir , it seems like ur heading in the right direction , hopefully you will have a 750 fps 12 pump gun in the near future , then when u wanna go crazy pump to 20 and get 900s.
The 880 is wayy easier to start out with , shoots 700 out of the box.. But obviously , its pretty close to max potential out of the box also.. so even 200 fps was hard fought gains.. i think the gun maxed about 815 before any additional volume was added, i could be wrong.
This p17 would be my 3rd design if i continue, nice part is ill still have less than 100 bux invested in all 3 guns total. lol. If i do the third, I know now exactly what i need to do to have the effort real low , and the numbers really high.. Little things i learned like check valve spring pressure and geometry payoff huge for the next build..Heck , if i made 2 of the new design and sold one id break even on the deal.And if  we didnt experiement, and just talked about it , there would be nothing worth reading on any of the forums.It would just be " look at my 100 yard groups !" that no one really believes

[Rob M]

how will u fix the pump arm.?? some aluminum angle might do the trick>> if not super glue?? lol

[bhh]

And if  we didnt experiement, and just talked about it , there would be nothing worth reading on any of the forums.

Absolutely! I can only shoot so much before I get bored. When I'm experimenting it keeps me thinking.

I'm still chewing on my plans for a new pump arm, but anything worth doing is worth over-doing, lol, so I'll likely make it from solid aluminum. Or at least the part that connects the tube and piston linkage. A lot of it will depend on what I can find in the scrap pile, but I guarantee that it won't be the weak link. Lol.

[bhh]

It would just be " look at my 100 yard groups !" that no one really believes

And "it put three through the same hole once I threw out the two shots I pulled...."

[Rob M]

lol, i was gonna say , if you have some plate aluminum , u could outline the forst six inches of the arm , mill it shape it , etc.. then connect to the plastic part. Or , in the dream world , You could mill the entire pump arm from 6061! that would be a pretty cool looking piece with some mothers polish  . Anyway , what i was thicking was a steel backplate the lenght of the arm , lets say 1/4 plate bolted in 4 places to the arm.. then send a piecce clear through the plate, through the plastic , and drill the pivot hole in that ( i was thinking some threaded rod etc, make use of the steel backstrap somehow

[Rob M]

It would just be " look at my 100 yard groups !" that no one really believes

And "it put three through the same hole once I threw out the two shots I pulled...."

Im always shocked , Navy seal and army ranger snipers have great resumes, but end up on airgun forums (-'

[airgunner53]

i was more than excited to see the adjustable piston head, and even more excited to see the high levels of fps on an 880. i have been messing with my 880 for quite some time and did all the mods that i have seen on GTA. so far the mods that i did were
- smooth and reduce trigger pull (remove material)
- tape mod
- opened the transfer port slightly and polished it
- replaced seals with thicker o-rings
- flat top piston mod with JB weld
- and various other cosmetics
lately i also came to the conclusion that valve volume was a problem restricting performance so I began to increase volume. to do this I used a dremil and jewelers files and I took the valve piston and removed material by creating a ramp on the top (side view of valve) removing material from the base (nipple from casting) and creating a ramp on the bottom leading air up toward the transfer port. Also on the valve where the spring sits i removed material by creating a ramp on top and on the bottom to further direct the flow of air. on the inside before reaching the transfer port seal, i opened up the port to direct air above the transfer port seal rather than having the air fight its way around it. also I removed the stock bolt probe tip and replaced it with a longer and thinner probe to allow the pellet to be further in the barrel and reduce air restriction from the probe.

after this the volume increase I noticed more power. do not have a chrony, but i use a thick piece of hard particle board. before the volume mod a gold flight 8.9 pellet entered .110 at 6 yards after the volume increase it went .140 then came the stroke increase mod.
i did this by cutting .300 of the end of the piston arm above the rollers and shaved the pump arm link to curve and allow the piston to extend an additional .300 out of the tube. using jb weld and plastic from a soda bottle to sit agains the inside of the pump tube i extend the tube by an additional .25 for added air. some sanding on the fore arm above the pivot point and now I am able to compress more air. after this mod the same pellet was able to enter the same piece of wood .280 double what it did before.
to be able to dump all the air I use an additional 880 trigger spring under the loading port from where the flat spring is removed. with the reduction in trigger pull mod, the additional spring tension is not even noticeable.
very happy to see how hard it sinks allow hard pellets and it nearly obliterates lead pellets. I will post pictures soon of my mods.

[Rob M]

sounds great, im sure u have seen this  2000PSI  SHOT



910fps was the best i got , but i think it was shooting hotter when i had the old setup seen in the vid

[Rob M]

the 2 biggest restraints on the gun are swept volume , and valve volume. alter those to an extreme and you have a gun that makes a 397 look weak // the problem is the swept volume is not easily altered, and therefore the max power didnt come on till 23 pumps.

[airgunner53]

the result of my 880 with the additional .300 stroke resulted in .250 in tube additional volume resulted in what I think was me breaking the sound barrier with a PBA raptor at 24 pumps. at this point it no longer makes the hiss noise suggesting that it is no longer taking in air to overcome the pressure in the valve. my goal was not to shoot the fastest, but it was pretty impressive hearing that come from my little 880.

have you considered using an o-ring with a backup teflon seal for added pressure. or maybe a double lip o-ring that is used in hydraulics for pressures of up to 6000 psi.

[Rob M]

man id love to know what it does over the chrony/?? also , yes it will get pretty loud.. what is ur piston setup?? i dont recall.. I never had air getting past the piston , i simply started breaking pump arms first

[airgunner53]

sorry the piston is a stock piston with the dimple filled with JB weld and a thicker o-ring. the sound I was referring too is when you pump it and start to extend the pump it makes a hiss as it draws in air for the stroke. it normally will spring out a certain amount and more with each stroke as the pressure in the valve increases. toward the end of a max out pumping cycle the arm springs nearly all the way out. i pumped it until the hiss was no longer heard at extending the arm guessing that the piston was no longer able to overcome the pressure in the valve.

picture of my valve volume mod. basically was leading toward redirecting air rather than increasing volume but it results one in the same.
the added spring opens the valve ( to my knowledge) at the same rate despite the amount of pumps rather than using a single heavier spring.
I will post a pic of the extension of the tube and internals soon after my class ends and I have time to get back into it.

[Rob M]

very cool! usually when the balance was reached, and it no longer took on air ( hiss ) it meant that i could no longer build pressure on my gauge, and not pump past it.. ideally , a 2 stage would be used ( which i have a design for ) to kepp pushing air past the 2200 psi mark

[bhh]

Nice work, and nice rifle. Where did you get the metal parts? Old guns on ebay?

Looking forward to the pics.

If you are going to be a hard-core experimenter, you have to get a chronograph.