Author Topic: made an adjustable piston for the 880 (Read 48399 times))

bhh · « **Reply #100 on:** October 02, 2016, 05:29:29 PM »

I enlarged the volume some. I cut about a thread off the valve and cut a slight chamfer. I also machined a new inlet valve plug. It increased the valve volume by .25 cc. I put it and the old one (separately) in a graduated cylinder to measure their displacement.

So what do think happened?

Rob M · « **Reply #101 on:** October 02, 2016, 07:19:47 PM »

the velocity dropped pumps one to 15 , or it leaks?

bhh · « **Reply #102 on:** October 02, 2016, 07:43:31 PM »

It really didn't make much difference from the original valve. At 10 pumps it was 2 fps faster, and at 30 pumps it was 20 fps less.

I tried five different sized valves: -.9cc, -6cc, -.3cc, stock, and + .5 cc (estimate). Looking at all my numbers, there wasn't much difference in the last three sizes. Time to find a bigger and/or longer tube.

Rob M · « **Reply #103 on:** October 02, 2016, 10:21:11 PM »

thats really odd, seems to me the piston is no longer compressing air at a certain point.. how in the world it lost velocity at 30 pumps is beyond me. What percentage of the valve volume was added with the last mod, Im guessing by how small it sounds , that it wasnt significant enough to impact the velocity

bhh · « **Reply #104 on:** October 02, 2016, 10:56:09 PM »

Quote from: Rob M on October 02, 2016, 10:21:11 PM

thats really odd, seems to me the piston is no longer compressing air at a certain point.. how in the world it lost velocity at 30 pumps is beyond me. What percentage of the valve volume was added with the last mod, Im guessing by how small it sounds , that it wasnt significant enough to impact the velocity

I think I added around 15 percent. Maybe the o-rings are getting worn? I haven't changed them during this valve volume test series, so maybe they are getting worn?

Here are some pics of the action. If nothing else I could gain a little with a longer tube with a shorter cocking arm slot.

Rob M · « **Reply #105 on:** October 02, 2016, 11:10:03 PM »

the tube plays a big role in the chassis of the gun , unlike the 880// longer stroke means minimal gains , and changing the pump geometry, its actually harder to do on some models.. the toggle ( connects the piston rod to pump arm) would need to be elongated..WHAT I would do// Chop the tube 1 inch in front of where the valve connects. machine a derin rod step up with 4 orings. ( 2 on the stock tube side , 2 on the new tube side so .750 to 1 inch adapter) drill right through the adapter with 1/16th but to keep headspace at nothing.. make sure the adapter on the valve old tube side fills the void till its flush with valve face. now take ur pump assembly , add a 1 inch piston and transfer to the new one inch tube// Sounds complex but its a one day project .

Rob M · « **Reply #106 on:** October 02, 2016, 11:12:17 PM »

in other words , ur just doing a step up from the old tube which stays the same at the back, to the new tube which is big bore. the new tube and old tube would be pinned to the adapter, and the front of the new tube would be secured via the rigidity of the connection and perhaps stabilized by the barrel shroud

Rob M · « **Reply #107 on:** October 02, 2016, 11:13:56 PM »

my 1 inch pumper is pure beast to pump ( even with an extended sidelever) , so id probably shoot for .860 bore give or take

bhh · « **Reply #108 on:** October 02, 2016, 11:45:45 PM »

Sounds like a good plan. Do you think 1020 DOM A513 tubing would be strong enough? 1" od with a .065 wall thickness has an id of .870.

My tube has an OD of .908. I thought about turning down the short valve section to fit inside the 1" tube and then silver solder and screw them together.

Rob M · « **Reply #109 on:** October 02, 2016, 11:57:40 PM »

yes, the dom will be ample strong.. Your idea might impede the other functions of the back half of the tube..If not ( you know better than I ) go for it.. Dom is cheap and .870 sounds awesome.. Were it not for the sidelever long arm im not even sure i could pump my build more than 2-3 strokes lol.

Rob M · « **Reply #110 on:** October 02, 2016, 11:59:25 PM »

Whats the i.d of the stock tube , at .908 od the id must be closer to .800.

Rob M · « **Reply #111 on:** October 03, 2016, 12:02:20 AM »

i see how ur idea could work , but i see it as a lot of milling and drilling , and too much effort compared to turning an aluminum or delrin adapter, when u set aside the chopped section from the pump tube, do not forget to duplicate the stroke length and geometry obviously , u want to be as close as possible to the stock configuration, just bigger piston moving in a bigger tube

Rob M · « **Reply #112 on:** October 03, 2016, 12:14:06 AM »

im still trying to figure out why the gun isnt topped out at a lower pump count. the daisy is beyong difficult to pump at 17 and by 20 you can feel your no longer doing much to compress air, just burning calories.. Im wondering why its letting u pump 30 times before hitting max velocity.does it feel extremely hard to pump after a certain number

bhh · « **Reply #113 on:** October 03, 2016, 12:37:39 AM »

Quote from: Rob M on October 03, 2016, 12:14:06 AM

im still trying to figure out why the gun isnt topped out at a lower pump count. the daisy is beyong difficult to pump at 17 and by 20 you can feel your no longer doing much to compress air, just burning calories.. Im wondering why its letting u pump 30 times before hitting max velocity.does it feel extremely hard to pump after a certain number

Too funny. I was just thinking about this exact thing. Why is pump 40 the same difficulty as pump 15? Something is just not right. I wonder if I'm getting flex at the the back of the piston since it is made of plastic? Or... is the valve face slightly off perpendicular and leaving a wedge of air that can't be stuffed into the piston? I'm going to eliminate these two variables before going with the larger tubing. BTW, my current ID is close to .780 (can't remember exactly right now.)

Rob M · « **Reply #114 on:** October 03, 2016, 12:44:00 AM »

ive found with my pumper experiments ur dead on.. the yaw and cant of the piston assembly is ur biggest issue and the bigger the piston or the stiffer the piston , the more this becomes a problem.. tighten up every dam link, make the piston one long a's solid rod of aluminum.. or keep the adjustable screw section really dam short.

Rob M · « **Reply #115 on:** October 03, 2016, 12:45:55 AM »

No one believes it but on crosmans, even tihgtening up the play in the pump arm pivot of the tube leads to gains for this exact reason.. MAtter of fact the reason for the piston cup assembly is to avoid overpumping lol. The designers intended it so the piston will push no more air after X numver of pumps

Rob M · « **Reply #116 on:** October 03, 2016, 12:56:51 AM »

another idea while ur at it, u can extend the valve by simply getting a home depot coupler male to female in that thead , then turning it down on the lathe .. just as an aside

bhh · « **Reply #117 on:** October 03, 2016, 10:24:48 PM »

Quote from: Rob M on October 03, 2016, 12:56:51 AM

another idea while ur at it, u can extend the valve by simply getting a home depot coupler male to female in that thead , then turning it down on the lathe .. just as an aside

That's a good idea. This weekend I measured the thread pitch to see if it was metric. It is 1.0 mm. I didn't measure the diameter, but I know the o-rings have a 15mm ID, so likely it is a M14-1.0 thread. I'm searching for an adapter in that size but haven't had any luck yet.

Rob M · « **Reply #118 on:** October 04, 2016, 12:19:18 AM »

male to female 14-1.0, im looking

Rob M · « **Reply #119 on:** October 04, 2016, 12:25:19 AM »

i think its 1.5mm pitch , double check

http://www.ebay.com/itm/Steel-Adaptor-M14-x-1-5-Female-to-M14x1-5-Male-Fittings-HEX-22-L-43mm-1-7inch-/322245392493?hash=item4b0752786d:g:0tQAAOSw9IpXw8wg&vxp=mtr

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made an adjustable piston for the 880

Author Topic: made an adjustable piston for the 880 (Read 48399 times))