Author Topic: made an adjustable piston for the 880 (Read 48541 times))

Rob M · « **Reply #240 on:** December 06, 2016, 12:13:51 AM »

you could hook ur shop compressor to it.. 5 strokes or so ud be maxed out!!!!!!!!!!! precharge the tube to 100 psi (-'

bhh · « **Reply #241 on:** December 06, 2016, 12:46:12 AM »

Yea, that would be a learning experience. Just be hard to take in the woods.

bhh · « **Reply #242 on:** December 06, 2016, 12:47:38 AM »

Just a thought, but this rifle has a big fat magnetic probe on the bolt. I need to thin that down some.

Rob M · « **Reply #243 on:** December 06, 2016, 12:53:48 AM »

thats a huge factor along with the TP.. make one from steel rod , like coat hanger

airgunner53 · « **Reply #244 on:** December 06, 2016, 02:41:02 AM »

I did the same with my 880. took the barrel out and looked at the bolt to see how far it pushed the pellet into the barrel and noticed how the magnet probe was almost shrouding the entire transfer port. pulled it out and replaced it with a used rivet pin that was .030 longer and ground it down about 2/3 of what the original probe was. don't have a crony, but after this i did notice the pellet strike my target with a little more power such as watching my pellet trap rock a little harder.

I know high pressure is a concern of making the pellet fly faster but i believe that more volume would help keep the average pressure constant as the pellet travels down the barrel thus creating more energy and reducing the drag after pressure decreases as it expands in the barrel. a port job definitely helps to redirect flow to push the pellet.

bhh · « **Reply #245 on:** December 07, 2016, 06:36:16 PM »

Last night I used a dremel and reduced the diameter of the bolt probe. Today after work I shot of few over the chrono. Temp was about 35 degrees and the light was fading fast so I only shot one pellet at each data point.

10 pumps - 679
15 pumps - 750
20 pumps - 819
25 pumps - 866
30 pumps - 872
40 pumps - 907
50 pumps - 939

Rob M · « **Reply #246 on:** December 07, 2016, 08:30:52 PM »

holy &^^&@@@@@@@@@@@@@ we need a way to increase the swept volume.. One way is to machine the face of the valve down a few thousandths.. .060 or so so as to not compromise the burst

Rob M · « **Reply #247 on:** December 07, 2016, 08:35:38 PM »

what baffles me is theres actually gains in the last 10 pumps

bhh · « **Reply #248 on:** December 07, 2016, 08:58:39 PM »

Quote from: Rob M on December 07, 2016, 08:30:52 PM

holy &^^&@@@@@@@@@@@@@ we need a way to increase the swept volume.. One way is to machine the face of the valve down a few thousandths.. .060 or so so as to not compromise the burst

Actually, it's already been done. When I made the new pump arm I drilled the piston arm hole slightly farther from the pivot than stock. It ended up putting the piston about 1/16" farther down the tube. I faced the front of the valve by sanding it on a piece of glass. Bottom line - the valve face and piston face are flat, mate perfectly, and I have about 1/16" more stroke.

Rob M · « **Reply #249 on:** December 07, 2016, 09:00:15 PM »

okay.. we will have to find a way to get more volume ,. even if you could get 900fps off 20 pumps , it would be golden

bhh · « **Reply #250 on:** December 07, 2016, 09:05:54 PM »

I need a new tube. The slot for the pump arm is a half inch longer than it needs to be.

Rob M · « **Reply #251 on:** December 07, 2016, 09:13:30 PM »

Quote from: bhh on December 07, 2016, 09:05:54 PM

I need a new tube. The slot for the pump arm is a half inch longer than it needs to be.

new pump tube

what size steel , im not good at much but i can scour the web for delrin , steel and aluminum tube

bhh · « **Reply #252 on:** December 07, 2016, 10:06:44 PM »

It's metric. .907 o.d. x .787 i.d. Or 23mm x 20mm.

The trouble is I'm approaching the point of diminishing returns because I'm going to have all this time and money in a hopped up power plant that will be attached to a crappy plastic shell that won't hold a scope and has a crappy plastic trigger.

Rob M · « **Reply #253 on:** December 07, 2016, 11:07:21 PM »

those sizes are cheap.. but i see ur point.. Why no scope?? thats odd>id get to tuning it for 800 fps or more at 20 pumps.

Rob M · « **Reply #254 on:** December 07, 2016, 11:10:35 PM »

http://www.ebay.com/itm/6061-T6-Aluminum-Seamless-Tubing-OD-24mm-ID-20mm-Length-0-5m-1-64-ft-EA-D-/181557631286?hash=item2a45ad9936:g:wkMAAOSwFTRTrTy9

turn this down 1mm , you have a new longer comp tube and extra

bhh · « **Reply #255 on:** December 07, 2016, 11:30:11 PM »

The scope rail is undersize and normal rings don't fit. The whole clamshell is flexy too.

I think I'm going to reduce my extra valve volume a bit and see what happens. Since I'm using the tube as the valve wall I don't think I want to use aluminum. Although I don't know the burst pressure of aluminum. It may be fine.

Rob M · « **Reply #256 on:** December 07, 2016, 11:33:22 PM »

the burst pressure on that tubing would be roughly 3500psi .. (with 1mm or .040 wall. )

Rob M · « **Reply #257 on:** December 07, 2016, 11:35:45 PM »

believe it or not the thomas FT 3500 dollar gun has a 6061 tube , I know i couldnt believe it either. 2024 is way better for the application.. Anyway, yep, fine tuning.. the scope rail thing has me baffled. if you have some jb weld, id bed both insides off the clam , get it stiff. then use a weaver to 11mm rail adapter ( 3 bux on ebay) and mill one side down 1 mm to really grab the clamshell rail

bhh · « **Reply #258 on:** December 08, 2016, 12:25:50 AM »

Yea, the scope rail could be shimmed/epoxied to work fine.

I think that tube could be left full thickness in the area surrounding the valve and all the way to the pump arm pivot. It would only need to be machined where the trigger group and butt stock attaches. I'll have to think on it a bit.

Rob M · « **Reply #259 on:** December 08, 2016, 12:32:01 AM »

i know how you feel.. every 10 bux i put into the 880 killed me.. Heres the real reason.. After you strip and reassemnble these guns 20 times, theyre never the same.. If you took what ya know now , and applied it all at once to a new gun, getting a 15 dollar tube would seem like nothing.. I know exactly how it felt when people were asking, " why not 25 cal on the 880" .. well yea , 35 bux is nothing for a machine tool, but for an 880 its a fortune

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made an adjustable piston for the 880

Author Topic: made an adjustable piston for the 880 (Read 48541 times))