13XX/2289 Piston Testing

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....

http://i378.photobucket.com/albums/o...psf98378a9.jpg

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

http://i378.photobucket.com/albums/o...ps94dddaaf.jpg

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....

http://i378.photobucket.com/albums/o...ps436278b6.jpg

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

But no one has ever replied to this thread just say AWESOME! This data is priceless. It demonstrates that I can get the same as 10 pumps with only 6. Personnaly I think the ultimate pumper mod would be a two or three pump requirement to get 800-900 psi and thats it. No more pumps required and if it maxed out at that psi then it would be safe as well. I have to say thanks for sharing this and even taking the time to do it. To me, simply amazing and invaluable info.

I seem to have missed this when it was posted.

Only "problem" with the metal piston head and the Crosman-type pump cup is the machining on it would be a bit more than the o-ring version, and then one is "married" to using the Crosman cup afterwards.