880 Adjustable Piston

[Ronno6]

Perhaps you could drill the area to tap drill size while holding the O.D. in a 5C collet to make the hole round and easier to tap.  Then use green loctite to secure the stud.

THAT is the ticket!
Purchased a 5C collet in 10mm. The round section of the pump rod comes aboit 1/4" shy of the front, but the collet grips it fine.
I open up the ID a skosh and tap.........perfect!
The round section wouldn't project onto an ER32 collet as much; it would still grip, just not as well.....

Pretty much have it figured out now.
I will put a felt oil washer just behind the piston, and the assembly should last a lifetime.

Looks as though these well be relegated to super tuning 880 etc. rifles for resale.
This pump assy. and an O-ring bolt should make the 880 platform about as good as it can get.
First gen plastic pump arms will be upgraded to less flexy later design plastic or ideally metal arms.

[Rob M]

heres what I made in 2016 , similar principles.

https://www.gatewaytoairguns.org/GTA/index.php?topic=113523.0

[Ronno6]

Mr. Rob,
I remember seeing this some time ago.....I know the idea wasn't original.
But the technique is, somewhat.

Tapping the ID of the pump link really simplifies things.
The piston needs to have the stud loctited to the piston, as there isn't room for a lock nut at piston, and at the linkage.
Should hold adequately, especially if the stud is securely tightened into the piston.
Then the adjustment will be done at the linkage. This leaves room for a felt oil washer
punched with a 5/16"ID hole for direct placement over the stud.
I could machine the lock nut a tad thinner,and leave a neck around the threaded hole at 3/8" dia to use a stock sized oil ring......
Or a backup washer over the neck...... the mind just keeps working...
Ideally there should be a washer to back up the oil ring, but there isn't room with the piston length i have used so far.
The piston for the new style could be shortened, but not for the old style, that needs the projected face with the convex rim.

So far I have only made a piston with the O-ring groove minor diameter .005" larger than stock.
Is there an optimum minor diameter for this groove?

I wonder if this would prove to improve performance of the 1977?
Certainly the pump spring of the 1977 gives prior to seeing 2000# of pressure.......(?)

[Rob M]

I think the factory piston Specs are about optimal , but it needs to be stabilized( In all directions of flection)  somehow as you alluded to..  The valve fired just fine at 2000 psi , but when i started modding the trasfer port the poppet extruded as I recall. Another thing i remmeber was snapping a few pump arms in the higher pressure range , once in a while a metal pump arm comes up on ebay , i never scored one but its a nice touch. I didnt have the oil wiper , so thats a good improvement ..  I think looking back I would have simply optimzed for max power at 10 pumps , and ignored the numbers beyond that ( even though thats where most of the gains seemed to happen ,, I remember someone stacked washers under the stock piston and also got very good numbers.

[Ronno6]

I have seen people wrap teflon tape or dental floss in the O-ring groove and see improvement..
I have also seen some stock pistons with rounded bottom grooves and some with square bottom grooves.....
Not sure of performance differences due to that.

I played with stacked washers, with solid and with a couple Belleville washers for some give,
but those attempts were all non-adjustable and used stock, single O-ring pistons.

I have machined 1 jam nut leaving a neck for a washer. Works well, but involves
additional machine operation and may not be a necessity....

I do believe that the tapped pump link is the simplest system.
Too bad Daisy does not sell it solo, but the assy isn't that expensive......

The old brass tube 880's have the metal pump arms.
Those models benefit from a piston shapes like the original, with the convex rim/extended face to
project into the swaged tube ring and contact the seal disc, eliminating dead space between the 2 faces.

[Rob M]

yes, the tapped pump link sounds like a good solution.. cant recall how i had it arranged.. I always wondered if reducing the valve volume would improve performance in the lower pump count range, I think it would but with diminishing returns.. ( after 5-6 pumps im thinking the power gains would taper off.

[Ronno6]

yes, the tapped pump link sounds like a good solution.. cant recall how i had it arranged.. I always wondered if reducing the valve volume would improve performance in the lower pump count range, I think it would but with diminishing returns.. ( after 5-6 pumps im thinking the power gains would taper off.

Your piston stud employed the OEM style roll retaining cross pin and a collar on the stud to limit the insertion depth into the linkage.
I had considered something similar...see my post #10

Daisy has altered the chamber volume over the years.
The later design uses a seal abutment with a large nose that protrudes into the valve body.
That does take up some volume... Those guns really give a nice "crack" report.

The center valve in the photo is from a 922.
Notice the ramp in the throat of the valve.
There are other valves that actually have the front of the throat partially blocked........... I don't currently have a pic of that...

There is one older design that I believe had a larger volume...I believe it was the early 881.
The throat of the valve body was larger..... The interesting thing about this system is that, when  when pumped
and dry fired, the report is a dull "thud" as opposed to the "crack" that is more typical.
I always attributed this to the increased chamber volume....
However, when firing a pellet, that design does give a satisfying "crack..."

[Rob M]

I see what you mean , and yea maybe some operated at lower pressure more volume.. In my experiments with the p17 blow open valves , I noticed that just an increase in spring pressure opening the dump valve produced a shaper report. So the time for the air to escape or lock time may be a factor

[Ronno6]

OK
Here's the finished Alpha models for the New Style (right) and Old Style adjustable solid pump pistons.

I have contacted a couple of members to be testers for each style............
The piston for the NS may eventually be shortened, or the O-ring moved closer to the face.
Dunno if that is critical or not.

Instead of a jam nut, I am using a full nut and turning a shoulder down to 3/8" dia so that the nylon and felt washers will locate on it.

the length from the locknut/linkage interface to the face of the piston has been set at .824". which is what I measured on a stock assembly.
Of course, that dimension cam be fine tuned (by design......)
I may ultimately switch to fine threads to make adjustment less coarse.....

Always a better mousetrap.............

[Ronno6]

This mod has 2 aims:
1.Eliminate the elastomer spring.
2.Enhance performance.

Most shooters of the 880 are totally unaware of the problems resulting from the deficiencies of the ES.
They pump, it shoots. Performance decline due to the degradation of the ES is so gradual as to be imperceptible. Most shooters do not even know that the ES should be there or what it should do..I have resealed many 880's in which the ES is just....gone!

2. Performance enhancements may be modest below 10 pumps. (Field testing to be done.)The ability to pump higher pressures can be achieved with pump reps past 10, a routine that is not for everyone.
And, plastic pump arms are flexible..........and flex increases at higher pressures.....
The older metal pump arm 880's will benefit the most.
The double O-rings help in this regard.

A nearly identical result can be achieved by replacing the ES with a stack of flat washers.....however, as the thickness tolerances of washers varies greatly, mixing and matching washers to arrive at the proper stack height will be critical and tricky.

All in all, it has been an interesting exercise that has yielded a nice part, but I don't believe it will see much demand by the mainstream of 880 shooters.......

[caudie]

Just received Ron's new Piston Assembly this morning.  I'll install it as is and go from there to adjust it (if needed)..  I'm looking forward to installing it and testing. 

I used 880#4 to test Ron's piston assembly.  Just a simple swap of piston assemblies and a good oiling, it was good to go.  After shooting 10 pellets for warmup, the first 5 shots through the Chrony at 10 pumps avg 737 fps.  About an hour ago this morning I fired a 5 shot string with this same gun with the solid stack of washers in it and it avg 706 fps with the same 7.8 gr PK pellets.  I'll Chrony 4,6, and 8 pumps with it soon but this is an impressive start.

[Van]

  I also received Ron's new adjustable 880 piston yesterday. This gun is an old style 880. The piston is very well made and matched my old piston length exactly, so I just swapped it in without doing any adjustment.  I fired a few shots before the swap with Crosman hollow points and got 619fps at 8 pumps, 674fps at 10 pumps and 698fps at 12 pumps.  After the swap I got 629fps at 8 pumps, 680fps at 10 pumps and 705fps at 12 pumps.  I only plink targets with it, so I was not going to try adjusting it for max FPS. It will probably pick up a bit of speed as it wears in. I just did not like the ES part that degrades over time. It's a nice improvement if you plan to keep the gun and want it to hold up a long time. 

[caudie]

                                                        All 5 shot strings    PK 7.8 gr pellets     18 Oct 2024
                                 Ron's Piston          My 1st Flat top              My 2nd Flat top         
20              10p          725  fps                     698 fps                              706 fps
30                8p          674 fps                      688 fps                               669 fps
40                 6p          621 fps                     624 fps                               608 fps
60                 4p          532 fps                     542 fps                               525 fps

The Flat Tops I mention above were done with a layer of JBWeld to fill  the hole in the top of the pistons.  The 880#4 has the OEM piston with the hole in top.  So not so much apples vs apples but it is a comparison of a stock gun and this one.  It started out shooting 655 fps when I first bought it.  But I wanted to compare it to the Flat Tops I have.  Ron's piston still beats them,  at 10 pumps.  I can't quite feel it hit the Abutment when I close it, so a slight adjustment fix that to take up slack  (if any).  I'll find out when I try it.

This morning after toast and coffee, I shot a couple of strings through it at 10 pumps - 732 and 728 fps.  Time to try the adjustment on the piston.  I mean that's what it's for - Right?  Pulled the assembly, found a 1/2" wrench, loosened the nut and turned the piston out a little.  That was too much so I turned it back in some.  It felt just right to me.  Tried another string and got 725 fps avg.  That's still putin em out with authority.  I would rather have the piston adjusted right then the extra 12 fps.  I think I'm going to leave it right there.

All 4 880's I have develop the most power with heavier pellets.  So I want to try the WRN 9.8 gr in this one.  I'm working on the last tin of these so I'm getting pretty stingy with em, but this I'll try.  My two Flat tops put out 9.2 and 8.9  FPE with these pellets. So time to try em in this one.  The WRN 9.8 gr (the old style - no longer available)  at 10p shot an avg. 663 fps - 9.6 FPE.  I have an opened tin of Benjamin HP 7.95 gr pellets so I thought, what the hay, might as well shoot a few of those also.   The Benj's at 10p shot an avg of 718 fps - 9.1 FPE.  I'd pick either of these if I was still hunting.

I'd say that Ron nailed his objectives with this setup.  Eliminated the Rubber Bumper (Elastomer) and to enhance the performance.  Good work Ron, I like it a lot.  Anyone having a Daisy 880 that wants to eliminate that Elastomer Spring and/or wants an increase in power would do well to try this mod.  Another plus is it's easy to install, no compressor needed, just an easy swap of the whole piston assembly.  It only takes a philips screw driver and a very few minutes to install.  Remove 2 screws, one in each forearm panel and remove one pin in the linkage, no punch needed.  Remove the screws first on both sides, slide them both together forward a little and remove. To reassemble reverse that, both together.  That will eliminate prying them apart at the front, where a tiny plastic piece protrudes to  aligns them.  Any one can do it.