Author Topic: made an adjustable piston for the 880 (Read 48501 times))

Rob M · « **Reply #300 on:** January 09, 2017, 10:52:31 PM »

sounds good.. YES the hammer opening faster will be a good improvement.. I think the added volume may help were it reaching the barrel fast enough.. tough call.. since when u started the added volume seemed to help.. But now that you are building higher pressure it doesnt help.

bhh · « **Reply #301 on:** January 09, 2017, 11:38:05 PM »

I see what you mean about getting rid of the air faster. I'll open up those six holes a little and see what happens.

I took it apart a few minutes ago. Looks like I used a little too much oil. I cleaned it up and it seals fine now. I did notice that the plastic ball that seals the exhaust port is now getting egg shaped. The heat and pressure is distorting it. I don't think it was designed for this.

Rob M · « **Reply #302 on:** January 09, 2017, 11:45:52 PM »

haahahahaaa.. a guy on the green years ago made a 2 stage pumper pistol , needless to say it would pump to 6000 psi.. But he could never use the power because the check valve would start to melt beyond 4500 psi

Rob M · « **Reply #303 on:** January 09, 2017, 11:46:29 PM »

id bet right now you are pumping to 2000psi or so

bhh · « **Reply #304 on:** January 16, 2017, 08:36:53 PM »

I think this is the last gasp for the NXG.

I opened the transfer ports I drilled to enlarge the valve volume and installed a stronger hammer spring. It's really hard to cock now.

It didn't change much. I experimented with more pumps and with 30 pumps I got one shot of 897 fps followed by a second shot of 591.

I then experimented with changing valve volume by removing and adding o-rings. Larger volume retained air at a lower number of pumps and smaller volume gave reduced velocity.

I finished the tin of 500 pellets I was using for testing. I'll bet I averaged at least 20 pumps per pellet. Plus I tried a few other brands/weights.

Now my new aluminum pump arm has slop where the piston linkage attaches. So now I have excess headspace. The exhaust ball valve is a grooved egg shape instead of a ball. I think I've pushed it hard.

I learned a lot. Time now to see what I can get out of the 880 in as few pumps as possible. I think I'm going to set my limit at 12 ft lbs. Tubing should be here this week.

Rob M · « **Reply #305 on:** January 16, 2017, 10:29:31 PM »

hahahaa, im wondering how the heck the npx was retaining air , thought it was a dump valve?? anyway, i was pushing 14 fpe or so with the 880, 12 would have been fine..the biggest challenge u face is installing the new comp tube , not only getting it in there but getting it to seal to thee abutement and the valve face! thats why i was helbent on getting the .732 tube in stainless, it required no mods at all

bhh · « **Reply #306 on:** January 16, 2017, 10:48:38 PM »

Quote from: Rob M on January 16, 2017, 10:29:31 PM

hahahaa, im wondering how the heck the npx was retaining air , thought it was a dump valve??

Not sure what you call it, but it's the white ball thing here:

Rob M · « **Reply #307 on:** January 16, 2017, 11:43:19 PM »

thats a traditional crosman like valve.. cut a few coils off the valve spring and watch the velocity skyrocket !!!!!!!!!!!!!!!!!

Rob M · « **Reply #308 on:** January 16, 2017, 11:44:13 PM »

u could even put your stock hammer spring back in and see huge gains, No wonder there was confusion about the velocities.. 1000 fps should be easy

bhh · « **Reply #309 on:** January 17, 2017, 12:26:47 AM »

Quote from: Rob M on January 16, 2017, 11:43:19 PM

thats a traditional crosman like valve.. cut a few coils off the valve spring and watch the velocity skyrocket !!!!!!!!!!!!!!!!!

I don't think there is much to be gained there. I made a new intake plunger and it reduced the spring preload. There is barely enough pressure to push the cup seal back in place. Here's the stock intake plunger compared to the one I made:

Rob M · « **Reply #310 on:** January 17, 2017, 12:27:52 AM »

weird.. even with the heavier hammer spring it wont dump the full charge?

bhh · « **Reply #311 on:** January 17, 2017, 12:38:40 AM »

Nope. I just looked at it and I think I can clip one coil off the spring. I think I can sand down the diameter of the ball too. Maybe that will help airflow.

Rob M · « **Reply #312 on:** January 17, 2017, 11:16:38 AM »

yep, anything to increase flow and duration of the valve being open.. Kinda not worthy its serving both purposes

bhh · « **Reply #313 on:** January 17, 2017, 11:05:53 PM »

Ok, I clipped a coil and sanded the ball down to the smallest diameter I could. The spring barely compresses when I put everything back together.

Not much difference. It will hold another pump or two before retaining air. Velocity is about the same, but I didn't have the same pellets so I calculated energy and worked backwards to extrapolate the velocity as if I had been using the CPHP 7.9 gr. As it was, I was getting 804 fps using Winchester 9.7 gr domed.

But it's worn out. Piston linkage hole in pump arm continues to grow and headspace is increasing. I put all the parts in a box and I'm going to use the barrel for another project. I got a lot of fun out of the $29.27 this rifle cost.

Rob M · « **Reply #314 on:** January 17, 2017, 11:18:43 PM »

you really stretched that 30 bux, literally and figuratively !

bhh · « **Reply #315 on:** January 17, 2017, 11:28:58 PM »

...I didn't mention that I burned through almost that much in o-rings...Lol

Also forgot to say... Thanks for your helpful advice!

Rob M · « **Reply #316 on:** January 17, 2017, 11:33:39 PM »

NO problem.. this thread is not dead, U need to make a nice piston for the 880 and break 910 fps

bhh · « **Reply #317 on:** January 18, 2017, 12:08:19 AM »

Yep. But my goal for the Daisy is fewer pumps to 12 ft lbs and end up with a gun that will last.

Right after I put the NXG pieces in a box I got the 880 back out and took it apart. My tubing arrived today and now I'm in the thinking stage.

Rob M · « **Reply #318 on:** January 18, 2017, 12:19:32 AM »

lol, my numbers are well recorded, ill be able to compare our 15 pump notes! with the new tube you will need a piston anyway, no?

bhh · « **Reply #319 on:** January 18, 2017, 10:41:32 PM »

Quote from: Rob M on January 18, 2017, 12:19:32 AM

... with the new tube you will need a piston anyway, no?

Yes. I'm going to make a double o-ring but keep the Daisy rubber bumper design in some form. I think the zero headspace I used before caused the rapid pump arm wear. It just takes a thin film of oil on the piston face and suddenly I have an interference fit.

Anyway, one of the negative points of the 880 is that it uses cheap, soft steel for the frame.

But one of the positive points is the cheap, soft steel....

So what you see here is a .870" I.D x 1" O.D. pipe stuffed inside the frame. I heated and bent the bottom lips out straight, then used a punch to make an arch in the top of the frame, and then hammered the tubing inside and formed the steel into a tube around the new tube. I didn't use the actual tube for all the blacksmith work, but a piece of thick wall tube with the same O.D. instead. What you see in the pic is the actual tube.

Yea, I'll have to do a lot of fitting of the parts that attach, machine an adapter to fit the original valve, bend and tweak the frame some more, and maybe it won't work at all....

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made an adjustable piston for the 880

Author Topic: made an adjustable piston for the 880 (Read 48501 times))