Author Topic: made an adjustable piston for the 880 (Read 48380 times))

Rob M · « **on:** August 20, 2016, 02:15:56 AM »

after making 2 flat tops , i couldnt figure out the ideal stroke range.So with this its took me 5 minutes to get dialed in to where my harder shooting non adjustable is .. the pump arm is flexing by pump 6, and im pretty sure im maxing out a lot of components i need to reinforce. Took all night to make it , revise it and so on..i never thought an 880 could bark this loud with any mod
The .250 rolled rod on the pump linkage has an aluminum shaft going all the way to the roll pin threaded in 10-24.. thats adjustable.. the piston end also has 10-24 threads but will be red thread locked tonight.. after numerous shots the piston doesnt rotate back in the threaded rod , soonce its set and iit the tube , it doest change position.. Boring topic perhaps , if someone wants to test my other piston design and post numbers on it , u need a chrony, a stock 880 and some spare time ( and some integrity to actually post and nOT JUST TAKE THE FREE PISTON_)

FINAL NUMbER 910FPS 23 PUMPS 7.9 ( for now)

Underdog · « **Reply #1 on:** August 24, 2016, 03:46:30 PM »

That's pretty nifty. Have you measured the increase in FPS yet?

Rob M · « **Reply #2 on:** August 24, 2016, 04:18:21 PM »

heres a video of it firing at 2000 psi.............910FPS WAS FINAL 23 PUMPS

i shipped one to avator( bill) to test out and chronograph, he only has the regular flat top , not the adjustable so im awaiting his results today or tommor whenever he has time. The gains as u see in the video are huge with adjustable

Underdog · « **Reply #3 on:** August 24, 2016, 04:41:26 PM »

That's quite an impressive noise... And that's just a youtube vid- which never does the sound justice.
I have a video of a .50 Quakenbush booming away, and the video doesn't do it justice AT ALL.

Rob M · « **Reply #4 on:** August 24, 2016, 05:54:01 PM »

yes sir , it sounded like a 22 rimfire in real life lol. I may sell a select few after avators test run, im hoping he gets about 775 on 10 pumps , and over 900 fps on 20.. ive been making em non stop, testing different designs. Im also in the process of planning( read building) a long stroke conversion KIT ( more air per stroke , longer tube , different pump geometry) this way when u want 10 pump power , you only have to pump 6 time ( 47percent swept colume gain )

Rob M · « **Reply #5 on:** August 24, 2016, 06:18:31 PM »

Your prob thinking , why, whats the end goal lol.( its a 40 dollar gun) Well , 880fps , 10 pumps and reliable with about an extra 40 invested is the goal.. There is nothing i know of on the market like that , and i think the 880 is the perfect platform.. Most people say it wont be reliable. but so far there is no evidence of that.

Underdog · « **Reply #6 on:** August 25, 2016, 06:19:49 PM »

I think it's a good platform. Tom (StonyKill) convinced me of that. I was getting pretty good with mine until I screwed up the trigger and bought an HW95... and then a Gladius... and inherited a 1322, and haven't messed with it since.

Time to get back to the 880 and fiddle with that trigger again.

Rob M · « **Reply #7 on:** August 25, 2016, 07:09:05 PM »

The fine polishing of the trigger is a great mod but from what im gatheting if you want mega power the flat spring aids in the hammer release. Meaning u will get valve lock at some point thats yet to be determined. Mine fires at any pressure ..but the trigger isnt modded. Others report valve lock at 15 pumps with the stock piston.. Chrony numbers saturday night.

Davo · « **Reply #8 on:** August 27, 2016, 07:40:54 PM »

How are those chrony numbers?

Rob M · « **Reply #9 on:** August 27, 2016, 09:00:48 PM »

Rob M · « **Reply #10 on:** August 31, 2016, 11:07:22 PM »

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

bhh · « **Reply #11 on:** September 17, 2016, 06:51:31 PM »

Quote from: Rob M on August 20, 2016, 02:15:56 AM

after making 2 flat tops , i couldnt figure out the ideal stroke range.So with this its took me 5 minutes to get dialed in to where my harder shooting non adjustable is ..

Hey Rob,

What was the final optimal amount of clearance between the piston head and the valve?
I'm going to make a metal piston for a Umarex NXG pumper and may just make it one-piece instead of adjustable.

Thanks.

Rob M · « **Reply #12 on:** September 17, 2016, 07:14:41 PM »

the one piece was harder to get right , the dimensions of the piston i sent to BIll ( avator) were the closest to ideal ..You could email me since that was the last of the one piece pistons i made at the time

bhh · « **Reply #13 on:** September 17, 2016, 07:51:17 PM »

Quote from: Rob M on September 17, 2016, 07:14:41 PM

the one piece was harder to get right , the dimensions of the piston i sent to BIll ( avator) were the closest to ideal ..You could email me since that was the last of the one piece pistons i made at the time

Thanks, Rob. The Umarex piston isn't the same as the Daisy one so I'll have to get my dimensions from the stock piston. I am looking for the amount of clearance between the piston and the valve at the bottom of the stroke. I didn't know if they should touch, or if a few thou clearance is needed.

Rob M · « **Reply #14 on:** September 17, 2016, 09:23:02 PM »

from what i found to get max stroke, the lever doesnt wanna full shut with ease on the first stroke , i dont know the exact stroke distance.

bhh · « **Reply #15 on:** September 18, 2016, 12:04:39 AM »

Quote from: Rob M on September 17, 2016, 09:23:02 PM

from what i found to get max stroke, the lever doesnt wanna full shut with ease on the first stroke , i dont know the exact stroke distance.

Ok, thanks, that's about where I ended up tonight. My version is the 490 fps restricted output rifle. As such, it had 0.175" clearance between the piston and valve. I found a piece of UDPE and screwed it to the end to take up the space. I then machined it down to where the lever would lightly snap close.

It's not a metal piston, but this was quick and easy and it's fun to experiment. I'll build a proper metal piston later and compare the results.

Sorry for the "non-Daisy" thread hijack, but your results are encouraging and I want to see how it works with the cheap Umarex I bought.

Rob M · « **Reply #16 on:** September 18, 2016, 12:09:25 AM »

sounds good , no worry about the thread.. post results when you get them,/.Im sure there are serious gains to be made on that version

bhh · « **Reply #17 on:** September 18, 2016, 09:05:58 PM »

Results with 8.5 grain Crosman wadcutters:

10 pumps = 570 fps
15 pumps = 630 fps

Then I blew the seals out when I went over 15 pumps. They were not the original metric o-rings, but .75" o.d. ones I bought at the local hardware.

Time now to research the best o-rings and build a piston like you have done.

Rob M · « **Reply #18 on:** September 18, 2016, 10:51:12 PM »

so thats a big gain over stock.. Few hints, double orings, and more oring radius contact with the tube wall..

bhh · « **Reply #19 on:** September 18, 2016, 11:26:11 PM »

Quote from: Rob M on September 18, 2016, 10:51:12 PM

so thats a big gain over stock.. Few hints, double orings, and more oring radius contact with the tube wall..

Yep.

One problem I see is that the valve body OD uses two o-rings spaced an inch apart. A double o-ring piston head won't do much good if the valve body OD can't hold the pressure the piston can provide.

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made an adjustable piston for the 880

Author Topic: made an adjustable piston for the 880 (Read 48380 times))