valve options

valve options

Joined: October 11th, 2011, 6:25 am

May 20th, 2012, 8:32 am #1

what are the different aftermarket valve options for my 760. i would like your guys opinions as well as finding out what is out there.
thanks
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Joined: April 28th, 2010, 12:23 am

May 20th, 2012, 4:16 pm #2

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Joined: October 11th, 2011, 6:25 am

May 20th, 2012, 4:23 pm #3

whats the deal with it and how is it adjustable
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Joined: August 3rd, 2008, 2:29 pm

May 20th, 2012, 8:41 pm #4

it is a few pieces joined by a threaded piece so it can be made longer or shorter for the proper adjustment [/IMG]

TOO many freaks and NOT enough circuses
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Joined: April 23rd, 2012, 11:10 pm

May 20th, 2012, 10:08 pm #5

Having studied this a bit, and checking with a few engineers, ( not counting being one ) I can see no merit whatever in a flat piston over the conical cup **properly adjusted** save the bit of headspace in the intake port of the conical valve, easily resolved by stuffing that port.

I would ( and did ) put the effort into a nice stiff piston, clearanced to zero.

If I were buying parts, I'd go with the adjustable piston first and foremost.

but, that's me.
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Joined: October 11th, 2011, 6:25 am

May 21st, 2012, 12:23 am #6

what are the different aftermarket valve options for my 760. i would like your guys opinions as well as finding out what is out there.
thanks
all right another question. how can you tell the piston is properly adjusted and can i adjust the plastic conical one in the gun now.
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Joined: February 9th, 2006, 10:35 pm

May 21st, 2012, 11:27 am #7

There are some old tricks you can do to improve upon the original plastic piston. You can shim the pump cup out by poping the cup out of the piston, placing a small thin washer in the piston and put the cup back in. You'll probably have to file a bit off of the nose of the valve. Check out these threads about improvements to the piston. The 760 uses the same piston and practically the same valve as the 1377.

http://www.network54.com/Forum/275684/t ... ds+stage+I

http://www.network54.com/Forum/275684/t ... +Piston%21

http://www.network54.com/Forum/275684/t ... ck+2289---




I plink, therefore I am.
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Joined: April 23rd, 2012, 11:10 pm

May 21st, 2012, 1:33 pm #8

all right another question. how can you tell the piston is properly adjusted and can i adjust the plastic conical one in the gun now.
Follow the links airgunandy posted, and digest what's happening here.

The objective in doing anything with the pump is dynamic head space !
Due to flex in the materials, head space increases at increased pressures.
What Crosman gives you stock ( it is a $50 gun after all ) is just fine at 2 or 3 or 5 pumps.
Beyond that, the materials themselves begin to absorb some of the pumping energy, increasing with more pumps. That's one reason the stock gun tops out at relatively low energies.
Note that doing anything will increase stresses, and wear, on the parts.
You're trading longevity for performance, no matter how you slice it.

First, look carefully at the contours of the cup seal, and the intake end of the valve.
These need to be matched very closely. On the new guns, they are. Older guns apparently had a longer cone tip on the valve than in the cup seal. This caused the cup to seal off the intake port before all of the air could be squeezed into the valve, and so filing down the cone on the valve had benefits. That's no longer true. Check yours very carefully.

Once that's done, stuffing the plastic piston with drill rod, nails, screws, or similar, can stiffen the piston. This alone has no adverse affect beyond the work to do it. Fit the pieces carefully.
The piston will need to remain square under pressure, so it's important that all of the stuffing rods are exactly matched. Mine, I stuffed all 8 chambers in the piston with carefully fitted steel rod.

Next, shim the cup seal itself.
You want the cup to contact the valve **at the perimeter** first ! That's important !
So, an O-ring fitted between the edge of the cup seal and the end of the piston.
Then, a washer under the cup seal center, between the seal and the plastic piston, of the right thickness so that the outer perimeter of the cup contacts the valve either first, or simultaneously with the rest of the cup surface on the first pump or two.
It might appear that a steel disk would be better than a washer, but such is not the case !!
That very small space at the tip of the cone created as the cup seal deforms into the hole in the washer, if one is using a washer, allows the air to be directed to the intake port, and not merely trapped elsewhere and at very high pressure between the cup and the valve.

Now, when you pump, the cup seal perimeter contacts first, trapping air in the cup. At that point any head space outside the seal, such as around the valve but above the O-ring, becomes unrecoverable. Under pressure the cup seal deforms like a balloon. Closing the pump arm further, then squeezes that trapped air out of the seal and into the valve, much like squeezing a toothpaste tube from the bottom first, and progressing toward the cap. This is the effect you're after.

Because we are tinkering at the fringe, and we are already into deforming parts, the dimensions of shims are becoming quite critical. Don't get carried away. More is not necessarily better !!

Note that because there is actual physical contact between the cup seal and the valve surface, wear and stress is increased dramatically even on the very first pump. All three hinge pins, the linkages, and the valve retention are taking as much stress on the first pump as on the fourth or fifth, if you did it right and got the O-ring and washer the right dimensions.
This is not true of the popular flat top mods, as no physical contact at all is tolerable with that approach, therefore limiting the effectiveness of the flat top approach. ( and is why I see the flat top approach to be of limited merit )

For me, with the internals of the valve removed so that no air is compressed at all, the cup contacts the valve when the pump arm stock is just even with the edge of the trigger guard.
In this condition, the closing force on the pump arm is about 7 pounds.
Due to the mechanical advantage of the arm linkage, the force on the cup, valve, hinge pins and such, is considerably higher. ( I've not measured or calculated, but I would guess several hundred pounds )

All this does nothing for the head space actually in the valve, which includes the full length of the intake port, as well as whatever space exists inside the valve before the check valve surface.
I've stuffed the intake port with solid picture hanger wire, three strands, so managed to reduce that small space considerably ( proportionally ) also.
This has the affect of not only decreasing head space ( there is always a down side ) but also increasing the velocity of the air through that port, and therefore friction and heat ! Fast pumping theoretically can burn the cup seal and check valve in relatively short order.

Although not strictly necessary ( depending on how much fussing one wishes to do ) an adjustable length piston would make this considerably simpler to accomplish, eliminating all of the shims and piston stuffing, also allowing one to back it off and preserve the gun when maximum performance isn't needed or desired.
The only way I can see to improve on this, would be with an adjustable, contoured cup end, solid piston, and thinner cup seal, contoured to match on both sides. Adjustment would be even more critical, as the thickness of the cup seal is reduced, because the only safety factor would be how much crush the seal itself can take before something breaks. Reducing the thickness and crushability of the seal would reduce dynamic head space created as the seal deforms even further, but we're already tinkering at the edges, and would only really apply at high pump counts, and very high air pressures.
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Joined: September 26th, 2003, 2:20 am

May 21st, 2012, 5:26 pm #9

Having studied this a bit, and checking with a few engineers, ( not counting being one ) I can see no merit whatever in a flat piston over the conical cup **properly adjusted** save the bit of headspace in the intake port of the conical valve, easily resolved by stuffing that port.

I would ( and did ) put the effort into a nice stiff piston, clearanced to zero.

If I were buying parts, I'd go with the adjustable piston first and foremost.

but, that's me.
"Having studied this a bit, and checking with a few engineers, ( not counting being one ) I can see no merit whatever in a flat piston over the conical cup **properly adjusted** "

I know a few engineers and most tend to over analyze and rely on theory rather than experience. There is no replacement for experience when working with airguns. Flat top pistons do provide more power per stroke than the conical pistons. There is more to it than sealing and adjusting. Try it before you condemn it.

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Joined: October 11th, 2011, 6:25 am

May 22nd, 2012, 3:45 am #10

what are the different aftermarket valve options for my 760. i would like your guys opinions as well as finding out what is out there.
thanks
so i understand what curt said but it brought up another question. how is a flat top advantageous over the conical? ive heard that they are easier to pump, but i dont understand the physics behind it. btw thanks for all the help so far guys. ill try to borrow a camera and post pics of the project soon if i can.
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