Author Topic: made an adjustable piston for the 880 (Read 48417 times))

bhh · « **Reply #380 on:** February 16, 2017, 12:56:51 AM »

Lol. Ooops. Typo. I corrected my numbers. I had the fps wrong.

Back to second place behind you.

Rob M · « **Reply #381 on:** February 16, 2017, 12:58:42 AM »

14.53 fpe was my record, just checked. @ PHEWWWWWWWWWWWW, my walmart is 24.7 and 4 miles away , i was debating taking the wifes car

bhh · « **Reply #382 on:** February 16, 2017, 01:03:40 AM »

Quote from: Rob M on February 16, 2017, 12:58:42 AM

PHEWWWWWWWWWWWW, my walmart is 24.7 and 4 miles away , i was debating taking the wifes car

Lol. I have three more refurbs to experiment with...

Rob M · « **Reply #383 on:** February 16, 2017, 01:06:44 AM »

well , now you know exactly what needs to be done to hit 1000.. everything need to be rigid, need a metal pump arm from an older gun , the piston u have bottoming out on the abutement valve face, and some additional volume in the valve.. ANd perhaps a final addition to preload on the hammer .pretty simple formula with the big piston.. with the small one theres a few other tweaks needed to make 1000 , not many to get to 900 plus

bhh · « **Reply #384 on:** February 16, 2017, 08:20:58 PM »

I enlarged the valve by taking a little off here and there. I also opened the channel that connects the abutment area to the valve area below the transfer port.

Velocity was about equal at 10 pumps but lost a few fps below that and gained a few fps above that. By "a few" I mean single digits.

12 pumps and 9.7 gr Win domes = 748 and 12.05 fpe

Then I decided to go for broke (literally)

20 pumps, 10.66 gr H&N Barracuda = 786 fps and 14.63 fpe
20 pumps, 9.7 gr Win domes = 799 and 13.75 fpe
20 pumps, 7.9 gr CPHP = 881 and 13.62 fpe
22 pumps, 7.9 gr CPHP = 873 and 13.37 fpe
21 pumps, 7.9 gr CPHP = 854 and 12.8 fpe

See what is happening? The pump arm is giving out and power is going down as the hole is elongating and creating headspace.

Rob M · « **Reply #385 on:** February 16, 2017, 08:33:35 PM »

good numbers , but were back to rigidity in the linkage again.. By pump 14 u shouldnt even be able to pump without snapping the arm like a twig..whatever lil sneaky ways to get a solid stroke will make all the difference,. bushings washers whatever

bhh · « **Reply #386 on:** February 16, 2017, 08:47:59 PM »

Quote from: Rob M on February 16, 2017, 08:33:35 PM

good numbers , but were back to rigidity in the linkage again.. By pump 14 u shouldnt even be able to pump without snapping the arm like a twig..whatever lil sneaky ways to get a solid stroke will make all the difference,. bushings washers whatever

Yep, and I think the frame is flexing too.

Rob M · « **Reply #387 on:** February 16, 2017, 08:54:45 PM »

Quote from: bhh on February 16, 2017, 08:47:59 PM

Quote from: Rob M on February 16, 2017, 08:33:35 PM
good numbers , but were back to rigidity in the linkage again.. By pump 14 u shouldnt even be able to pump without snapping the arm like a twig..whatever lil sneaky ways to get a solid stroke will make all the difference,. bushings washers whatever

Yep, and I think the frame is flexing too.

yep.. my pump arm would flex past stroke 7, But i never really had problems with the rigidity for 2 reasons.1 becuse i was pushing a smaller piston , and 2 because I had an adjustable piston.. So as it broke in and created headspace I just adjusted it back out the 1/8th inch or whatever for max compression again... I guess a third reason I nevr had issues was i simply broke the pump arm first (-;

bhh · « **Reply #388 on:** March 11, 2017, 01:01:33 PM »

I just hit 15.24 fpe.

841 fps with 9.7 gr Win Domes,

with 23 pumps,

with only lightly modified stock parts - Not my big bore and not using a custom piston.

I made the piston flat-topped, added a washer to reduce the amount of squish in the piston bumper assembly, increased hammer travel, used a nut to add preload to the hammer spring, drilled the transfer port to .140, reduced probe diameter, reduced the leade in the breech, and opened the transition from the abutment chamber to the main valve chamber.

Amazing how all the little things add up.

Rob M · « **Reply #389 on:** March 11, 2017, 01:05:52 PM »

wow, id love to see pictures@ also , you need to spit a 7.9 outa there and see if you get 911 fps

if i still had mine , id certainly be outside testing with a 9.7 LMBO..

sounds like the 2 mods that made a huge difference were the TP and the hammer strength

bhh · « **Reply #390 on:** March 11, 2017, 02:06:05 PM »

Quote from: Rob M on March 11, 2017, 01:05:52 PM

you need to spit a 7.9 outa there and see if you get 911 fps

You know what they say, be careful what you ask for...because you just might get it. Lol

Here is a 7.9 CPHP with 23 pumps:

Rob M · « **Reply #391 on:** March 11, 2017, 02:08:43 PM »

holy COWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW, you now hold the official 880 speed record.. ID love to see these mods.. What i find most interesting is that you did not use a custom piston to acheive these results.. You should make a new post detailing the entire " how to " on the mods, and the phenomonal results.

Rob M · « **Reply #392 on:** March 11, 2017, 02:10:13 PM »

opened the transition from the abutment chamber to the main valve chamber. """

what does this mean?? i assume you machined the rear portion of the abutement

bhh · « **Reply #393 on:** March 11, 2017, 03:18:14 PM »

I have one more refurb 880 to go through. I thought I'd take pics at each step of the way and make a thread.

One thing to note is that the fastest shots happen with a fresh lube and a cold gun. As I continue to shoot it settles down to about 25 fps slower. On the other hand, I'm using a tube that had a scratch from the factory and I sanded it out by hand with 400 and then 800 grit wrapped on a stick. So the wall finish is not finely polished and no doubt a few thou bigger.

Most importantly is that 10 pumps gives over 10 fpe easily and without strain. Add 3 more pumps and it's 12 fpe. So it's a reliable, accurate rifle at that power level.

I did not machine the rear part of the abutment. I'll post a pic later.

Rob M · « **Reply #394 on:** March 11, 2017, 03:40:22 PM »

Sounds liek you found the ultimate power tune, and apparently no machining required

bhh · « **Reply #395 on:** March 11, 2017, 03:45:21 PM »

I figured it was the same as porting an engine: more flow=more power.

Sorry about the poor pics, but it was hard getting light down there and my phone cam to focus.

Stock:

Ported:

Rob M · « **Reply #396 on:** March 11, 2017, 03:47:19 PM »

so , you added a small bit of volume , increased the flow of that air, and then added hammer to get it into the barrel faster.. Awesome.. the .140 tp kinda finishes the job.. very cool

bhh · « **Reply #397 on:** March 11, 2017, 04:03:02 PM »

Quote from: Rob M on March 11, 2017, 03:47:19 PM

so , you added a small bit of volume , increased the flow of that air, and then added hammer to get it into the barrel faster.. Awesome.. the .140 tp kinda finishes the job.. very cool

Yea, that was my thinking. I'll bet you could add your adjustable piston and metal pump arm and hit 1000 fps.

Grampy ray · « **Reply #398 on:** March 11, 2017, 04:39:10 PM »

If you flatten the flooring wider (looking like a sideways D ) and put a radi on the edge like a waterfall it will flow even better Put a washer on the seat to protect it a from damage..i did the same porting on my 901.when I was looking at the port on mine I could see it was restricted by the poppet. There was little room for air to go down and around the poppet. Mine was way narrowed than the on in your pick it was as wide as the poppet.BTW do you have any picks of the pistion and rod that you modified.
I was not trying for a big power increase I was trying to fix my vertical point of impact that I was getting.i just went in and debur the portl a little.I might go back in and make it a little bigger and drill out the transfer port.
If I could figure out how to cut the seat lower so I would have more valve travel I think that could make some more power .I added the travel to my hammer already but I think it could use some more.

bhh · « **Reply #399 on:** March 11, 2017, 07:16:55 PM »

Quote from: Grampy ray on March 11, 2017, 04:39:10 PM

If you flatten the flooring wider (looking like a sideways D ) and put a radi on the edge like a waterfall it will flow even better

Put a washer on the seat to protect it a from damage..

Thanks for the advice. I was worried about the seat and stopped before I really wanted to because I was afraid of slipping with my Dremel.

Here's the piston. Just fill the center depression with JB Weld and then sand it flat until the step is gone:

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made an adjustable piston for the 880

Author Topic: made an adjustable piston for the 880 (Read 48417 times))