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 Post subject: 13XX/2289 Piston Testing
PostPosted: Sat Nov 01, 2014 8:44 pm 
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A while back I had a discussion about which was better, an adjustable metal Flat-Topped Piston and Valve or an adjustable metal piston carrying the original Crosman pump cup, meeting with a shortened (but still cone shaped) valve front end.... It is fairly well known that the tapered front on the Crosman 13XX/2289 valve is slightly too long, and hits the bottom inside center of the pump cup before the edge of the cup can hit the shoulder on the front of the valve, resulting in greater headspace than necessary.... Therefore, the first step was to machine 0.030" off the front of the cone on the valve so that the rubber pump cup touches the end of the cone at the same time the end of it touches the valve flange near the O-ring.... Once done, the pump cup conforms very closely to the front of the valve.... The idea of the adjustable metal piston is to put some pressure on the pump cup, squeezing it tight against the valve front to create minimum headspace.... This minimum headspace is already achieved, of course, with a flat-topped piston and valve because you have two flat surfaces touching....

I made an adjustable piston with two heads, one flat-top, and one carrying a Crosman pump cup.... I also had a stock plastic Crosman piston for comparison purposes.... In order to measure the pressure inside the valve, I drilled out the back half of a valve and threaded it 1/8"-27NPT for a 3000 psi gauge with 100 psi increments.... I machined up a Delrin spacer for inside the valve to provide a spring seat (in the original location) with a tiny (3/64") hole through it for the pressure to the gauge, and adjusted the dimensions until the internal volume of the valve was the same as a stock 13XX/2289 valve.... I shortened a 1377 pump tube just behind the stake that locates the valve (thanks to Skanzy for donating one to the cause and sending it to me) so that the gauge cleared the back, and added an 8-32 setscrew on the side to prevent the back half of the valve from turning when I mounted and removed the gauge.... The front trigger screw (actually a 1/4" long high tensile 8-32) was used to secure the valve in addition to the rear stake.... Here is a photo of the parts....

Image

and here is the assembly.... I used a stock 2289 pump arm and linkage, and a 3/16" solid steel pivot pin (easily removable)....

Image

I started with the stock piston and recorded the pressure at each pump to 20 pumps, then at 22, 25, and every 5 pumps up to 50 pumps.... I then removed the stock piston, adjusted the length of the one with the Crosman cup to put it under slight compression (you can feel the resistance when the handle is 1" from closing) and repeated the test.... I then removed the valve, replaced the valve front end with the flat one, installed the flat-topped piston (adjusted so that resistance on the handle is felt at 3/4" from closing) and tested one more time.... I actually ran each test 3 times and used the average, although the pressures were quite close on each run, basically within my ability to read the gauge (about 20 psi).... I then graphed the results, as shown below....

Image

Obviously, either of the adjustable metal pistons is a HUGE improvement over the stock one.... Using stock springs, a 13XX/2289 starts to retain air at about 1500 psi, which you can't reach with a stock piston.... The flat-topped design performed the best, although at 10 pumps and below there is very little difference.... in fact there is only about 1 pump difference at 12-13 pumps.... Above that, however, the more you pump the greater the margin for the flat-topped piston, which is what I expected to see.... As the pressure rises, even with the adjustable metal piston, the Crosman rubber pump cup starts to compress from the air pressure, and the headspace gradually gets greater and greater, lowering the compression ratio of the pump, and requiring more pumps to reach the same pressure.... It tops out at about 2400 psi at 50 pumps, which is 50% more pumps than the F-T-P takes to reach that pressure (~32 pumps).... I stopped at 40 pumps with the F-T-P (2520 psi), although the pressure was still building slowly....

I didn't test a "stuffed" piston, where metal rods are glued into the slots in the side of a Crosman plastic piston to stiffen it.... This mod is often accompanied by fitting an O-ring between the cup and piston, and a washer under the cup, to reduce the headspace, in conjunction with shaving the cone on the end of the valve.... Obviously this can't be any better than the adjustable aluminum piston using the Crosman cup which I tested.... It is just a cheap way to gain some benefit, how much is gained being in proportion to the care used.... It would be better than stock, for sure, and it wouldn't take much improvement to reach 1500 psi where you have to worry about retaining air....

I am very pleased with the results of these tests.... Not only do they clearly show the improvements that can be made, but they also give a plot of valve pressures inside a stock valve, which is a piece of information I haven't seen in this much detail before.... I hope you all find this information helpful....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
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PostPosted: Sat Nov 01, 2014 8:54 pm 
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Thanks Bob, quite the interesting read. I take to relieve pressure you just loosened the threads upon the gauge?

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PostPosted: Sat Nov 01, 2014 8:59 pm 
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Yuppers.... *grin*.... 1 turn out still had plenty of thread engagement and the air would slowly bleed out.... The threads still looked perfect when I was finished (I checked their condition every time).... Teflon tape is your friend....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sat Nov 01, 2014 9:23 pm 
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:lol: that it is :lol:

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PostPosted: Sun Nov 02, 2014 1:08 pm 
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Location: QC
Excellent, nice to see some actual numbers to illustrate the difference.


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PostPosted: Tue Nov 04, 2014 7:06 am 
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Brilliant. I'd been wondering what sort of pressure the 13xx run at.


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PostPosted: Tue Nov 04, 2014 11:07 am 
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I just ran across something I should have considered.... Our altitude here is 2500' (750 m) so the air pressure is only 13.4 psi instead of 14.7 which it is at sea level.... That, in theory, should require about 9% fewer pumps to reach the same pressure at sea level.... However that would apply to all configurations, and the maximum pressure reached would remain the same as that is a function of the flexing, construction, and headspace of the three piston types....

Bo

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Tue Nov 04, 2014 12:52 pm 
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Thanks Bob
I have found your work wit the 1377s very interesting starting with your Ubber pumper and now this , Showing what the real gains are.
Could you put this valve in the Ubber pumper to check the numbers there?


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PostPosted: Tue Nov 04, 2014 3:46 pm 
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Nope, the Uber-Pumper valve was modified to move back, sorry.... It has an F-T-P, however, and the swept volume is 24cc instead of 17, so just multiply the number of pumps by 17/24 to understand what is happening in the Uber-Pumper.... 17 pumps on the Uber-Pumper or Uber-Carbine (with the 2200 tubes) would equal 24 with the 2289 pump I these tests....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Tue Nov 04, 2014 9:07 pm 
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Location: Winnipeg, Canada
Nice to see a graph reflect previous personal experience, for a change :wink:

Not a whole lot to be gained going over 25 pumps.
Personally, I'm kind of leery of the 2-K plus pressures demonstrated on the graph- on an un-pinned valve,
but that's just me...

Regards,

Doc Sharptail

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PostPosted: Tue Nov 04, 2014 9:39 pm 
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The calculated safety margin on the single screw let me sleep at night, not including the support of the stake against the rear of the valve and the extra setscrew on the side.... I was using a 180K SCHS.... After the first run at 2500 psi I pulled the screw and inspected it and the hole in the tube and there was only a small mark in the tube wall from the thread and the screw looked perfect....

On the other hand, if you were going to be using over 2000 psi for extended periods, with the fatigue cycles to consider, I would agree that in addition to using a high-tensile screw at the front of the trigger, a pair of additional setscrews, one on each side, would be prudent....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Wed Nov 05, 2014 12:29 am 
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Thanks for the reply on the Ubber pumper


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PostPosted: Thu Apr 14, 2016 10:21 pm 
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FTP pulls into the lead at the first pump and just accelerates from there. It's an awesome mod if you are building a 3 pump per shot pistol or a 40 pump per shot beast.

Michael

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PostPosted: Sun Apr 17, 2016 6:24 am 
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great to see this , ive always thought about doing it . i never figured it would be as much difference . but you obviously got it right there . 2500 psi is pretty crazy pressure for a 2289


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PostPosted: Sun Apr 17, 2016 9:43 am 
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DerekVinyard wrote:
FTP pulls into the lead at the first pump and just accelerates from there. It's an awesome mod if you are building a 3 pump per shot pistol or a 40 pump per shot beast.

Michael


Holy cow, if you are pumping 40 times for one shot, YOU are the beast.


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