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Stuffing the HW40/P3/P17 piston

July 13 2017 at 9:52 PM

Steve in NC  (Login pneuguy)
AR&P

Reductions of the large headspace losses of the piston in this pistol has been experimented with by folks for years, using various methods, including the easy "aluminum tape" method mentioned recently by Lon (bigbore)...

http://www.network54.com/Forum/79537/message/1499870777

Unfortunately, only minimal velocity increases -- e.g., single digit fps -- have generally been reported. Here's one reason why.

A significant headspace volume -- of the order of 0.01in3 -- is represented by the HW40 piston O-ring gland...

[linked image]

Stuffing the piston so that it contacts the end of the tube at the completion of the stroke, traps this air and blocks it from timely entry of the transfer port when the gun is fired, resulting in a decrease in total useful charge and disappointingly small (if any) gains in MV.

Here's one solution -- going ahead and stuffing the piston, but leaving a diametric channel that connects the transfer port to the piston edge, so that otherwise trapped and wasted air has a way to enter the valve and provide useful propulsion.

[linked image]
[linked image]

A nice increase in MV (17%) and ME (36%) appears (shooting CPLs)...

Unstuffed: 372fps = 2.4fpe
Stuffed: 434fps = 3.3fpe

Steve

 
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michael wolfram
(Login marflow)
YF

how about a square profile o-ring and would two pin holes

July 13 2017, 11:49 PM 

drilled into the top edge of the piston work also ?????
mike

 
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Steve in NC
(Login pneuguy)
AR&P

The cross-section area of the communicating passage needs to be comparable...

July 14 2017, 12:02 AM 

...to the transfer port's, which ~2mm in diameter. That's a pretty good-sized "pin" hole.
Steve

 
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Scot Heath
(Login ish00ttrap)
YF

Very nice <nt>

July 14 2017, 12:10 AM 

.

 
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Erik
(Login hegshen)
YF12

What's the thickness of your piston head build up with the aluminum tape?nt

July 14 2017, 12:52 AM 


 
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robny
(Login robnewyork)
YF

A+

July 14 2017, 1:25 AM 

brilliant , well done sir.. the tp location is awfully inconvenient

 
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dan_house
(Login dan_house)
YC

The additional thickness on the piston face

July 14 2017, 1:24 PM 

negates the extra head space the channel creates?

How thick is the additional material?

 
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Steve in NC
(Login pneuguy)
AR&P

Not exactly. The added material (stuffing) increases pressure. The channel...

July 14 2017, 5:32 PM 

...while admittedly reducing the total volume of stuffing and therefore net pressure gain, provides access to the air hiding in the O-ring gland which matters more than maxing out pressure. Bottom line: Some stuffing is better than none, and 100% of the stroke volume is better than some. happy.gif

I'm still experimenting with stuffing thickness, but it's looking like the optimum likely lies somewhere between 0.033" and 0.045"
Steve


    
This message has been edited by pneuguy on Jul 14, 2017 5:37 PM


 
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michael wolfram
(Login marflow)
YF

Steve if you could find a happy thickness, could this be done

July 14 2017, 6:05 PM 

with an aluminum disc split and epoxy in place
17-18-19 gauge aluminum is right in that thickness range

my first post I was missing what was on the piston top now that I understand it better, it's a great idea


Steve have you ever look at Diana lp8 trigger setup ???????
mike

 
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Steve in NC
(Login pneuguy)
AR&P

Great minds think alike! That's (more or less) exactly where I hope this idea ends up.

July 14 2017, 9:25 PM 

Thanks.

Sorry, but I'm not familiar with the lp8.
Steve

 
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michael wolfram
(Login marflow)
YF

i found some 25mm and 22mm i8 gauge aluminum

July 14 2017, 10:13 PM 


 
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Lon
(Login bigbore)
YF

nt

July 14 2017, 10:40 PM 

[linked image]


    
This message has been edited by bigbore on Jul 14, 2017 10:44 PM


 
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Wyo
(Login WyoMan)
YF12

That's a pretty remarkable gain -

July 14 2017, 8:58 PM 

no doubt your ported channel idea allows for greater piston shimming (stuffing). Tolerances will probably vary, but with the conventional approach, my best effort was with a .025" shim and an o-ring change to U90D 117 (this is a little larger, oem is metric iirc):

[img][linked image][/img]

I never had much luck with foil tape as a shim. I think that the pressure, combined with the lube and moisture condensate, had their toll with the adhesion. But, it's great for testing...easy on, easy off. Well done!

Wyo


 
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Steve in NC
(Login pneuguy)
AR&P

Sounds like you found the optimum trade off between headspace reduction...

July 14 2017, 9:38 PM 

...(which gets better the thicker the stuffing shim) vs transfer port throttling (which gets worse the thicker the shim).

With a 0.025" shim, the spacing between piston and endplug is ~0.01", giving a curtain area around the ~2mm port that's only about half the area of the port itself. That's a pretty severe restriction of what's already a painfully small port, that will "go sonic" at an MV of only ~120fps!

So -- well done, yourself! happy.gif
Steve


    
This message has been edited by pneuguy on Jul 14, 2017 9:41 PM


 
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Longbow
(Login Longbow29)
YF12

On my fas6004

July 14 2017, 9:04 PM 

I made a new adjustable piston to play with the head space. I made a very snug print glad the fit the x ring I used. I reduced the piston face to gland to less than .010". So lost space was very small.
And it still didn't like 0 head space.

You could put a teflon back up ring behind the oring to reduce the loss

 
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Longbow
(Login Longbow29)
YF12

Print gland should read ORing gland nft

July 15 2017, 1:46 AM 


 
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Lon
(Login bigbore)
YF

I wonder how hard it would be to machine an optimized piston.

July 14 2017, 10:46 PM 


It looked pretty simple from the outside. I forget how the rod attaches.
[linked image]

 
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Steve in NC
(Login pneuguy)
AR&P

The P17 rod is threaded into the piston. The HW40/P3 piston and rod...

July 14 2017, 11:12 PM 

...are a single piece of cast zinc alloy.
Steve

 
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Lon
(Login bigbore)
YF

It looks easy...

July 15 2017, 1:46 AM 

I may try to machine one .04 over OEM OL. Are you going to be releasing any other mods like stuffing or shimming the O-ring groove? Any other ideas like only cutting the groove on the face half the length of the face(Radius) or tapering the groove so that it is not as wide at the piston face?
[linked image]


    
This message has been edited by bigbore on Jul 15, 2017 2:19 AM


 
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Steve in NC
(Login pneuguy)
AR&P

LOL! I do have a couple ideas for optimizing the shape/profile of...

July 15 2017, 11:32 AM 

...the groove/channel, but I don't think I'm keeping up with you! happy.gif

One thing I've experienced is that the extra force from higher pressure and piston/plug contact (especially when getting a little too enthusiastic about the amount of stuffing wink.gif) can cause frame flex that makes the breech seal more likely to leak if not in pristine condition.
Steve

 
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