Stuffing the HW40/P3/P17 piston

Stuffing the HW40/P3/P17 piston

Joined: May 12th, 2001, 1:29 pm

July 14th, 2017, 1:52 am #1

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...



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.




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

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

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Joined: March 18th, 2011, 6:24 am

July 14th, 2017, 3:49 am #2

drilled into the top edge of the piston work also ?????
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Joined: May 12th, 2001, 1:29 pm

July 14th, 2017, 4:02 am #3

...to the transfer port's, which ~2mm in diameter. That's a pretty good-sized "pin" hole.
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Joined: March 22nd, 2006, 5:11 am

July 14th, 2017, 4:10 am #4

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...



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.




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

Unstuffed: 372fps = 2.4fpe
Stuffed: 434fps = 3.3fpe
.
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Joined: February 1st, 2008, 4:11 pm

July 14th, 2017, 4:52 am #5

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...



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.




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

Unstuffed: 372fps = 2.4fpe
Stuffed: 434fps = 3.3fpe
What's the thickness of your piston head build up with the aluminum tape?nt
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Joined: April 28th, 2010, 12:23 am

July 14th, 2017, 5:25 am #6

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...



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.




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

Unstuffed: 372fps = 2.4fpe
Stuffed: 434fps = 3.3fpe
brilliant , well done sir.. the tp location is awfully inconvenient
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Joined: February 11th, 2002, 6:01 pm

July 14th, 2017, 5:24 pm #7

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...



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.




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

Unstuffed: 372fps = 2.4fpe
Stuffed: 434fps = 3.3fpe
negates the extra head space the channel creates?

How thick is the additional material?
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Joined: May 12th, 2001, 1:29 pm

July 14th, 2017, 9:32 pm #8

...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.

I'm still experimenting with stuffing thickness, but it's looking like the optimum likely lies somewhere between 0.033" and 0.045"
Last edited by pneuguy on July 14th, 2017, 9:37 pm, edited 1 time in total.
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Joined: March 18th, 2011, 6:24 am

July 14th, 2017, 10:05 pm #9

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 ???????
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Joined: July 4th, 2012, 7:51 pm

July 15th, 2017, 12:58 am #10

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...



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.




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

Unstuffed: 372fps = 2.4fpe
Stuffed: 434fps = 3.3fpe
<p>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):</p><p>[/img]</p><p>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!</p><p>Wyo</p>
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