Air Gun Home :: View topic

rsterne · Moderator

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....

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

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....

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

lampy · Veteran Member

Thanks for sharing your info, it gives me inspiration to start another project. I have an old 1377 with a sliding breech just begging to be modded.

nomadshooter · New Member

KOOL
NICE

Bob La Londe · Veteran Member

Bob, Thanks for the post. I've been playing with the idea of a high power pumper/acp for a while. This kind of tells me what might be practical.

fritz · Silver Status Member

Bob, for YEARS you've been doing this stuff, awesome tests, machining, calibrations, all that good stuff...

So I have to ask...

What's the best I can really do to my 1377 without breaking the bank? I never payed much attention to pumpers/pcp in years past because I only had springers.

From what I can find, looks like I need (max power and efficiency), is a 2x hammer spring, half power valve spring, flat top piston kit, bored/polished transfer port, and a steel breech kit.

And to make it nice, a roll pin delete, and weak trigger spring.

The bug bit me hard, so I'm already thinking about modding this girl. Free float a 18" barrel, LDC with front sight, LPA peep, carbine stock...

rsterne · Moderator

I've seen over 1000 fps with 7.8 gr. in .177 with a 24" barrel, and over 900 in .22 cal with 14.3s.... but it doesn't come easily.... That means larger swept volume pumps and lots of pump strokes, even with those....

I prefer working in .22 cal to take advantage of porting the valve, in .177 you really can't gain much doing that.... You can easily get 650 fps with 14.3 gr in a 14.5" barrel without pumping until your arm falls off.... probably closer to 700 with an 18"....

Bob

fritz · Silver Status Member

Interesting. I'd be happy with 700 fps shooting a 7.8gr .177. Don't really want to up to .22 on this gun. I'll eventually get a pcp setup down the road in probably .25.

Would you care to share a compilation of technical specs and part numbers? I could probably do a lot with just a drill press (can't fit a mini mill or lathe).

rsterne · Moderator

In .177, use the stock 0.140" transfer port.... You can drill the barrel port to match and deburr.... Leave the valve alone, except maybe round the bottom of the exhaust port where it enters the throat, but don't hog it out a lot, you can lose as easy as gain.... Your 18" barrel will give you most of what you want....

I took the sear spring and installed it in the valve (lighter than stock), and then installed a lighter spring in the trigger.... The stock hammer spring is all you need.... After that, the velocity will pretty much depend on how much you pump it.... A Flat-Topped Piston and Valve will reduce that a lot, of course.... With that, and the springs mentioned, you should be able to reach 14-15 pumps without the gun retaining air, and achieve the velocity you want....

Bob

fritz · Silver Status Member

After looking back at a lot of your posts, converting to .22 is almost a prerequisite for any kind of power. Keeping velocity high and trajectory flat is great, but raw FPE is much nicer, I know how to count my clicks or holdover.

Edit: I held on to the above for a few mins, before you posted. I think I'll do exactly as you suggest and likely be pleased as possible.

rsterne · Moderator

My .177 Uber-Pumper hit 706 fps with the above mods at (the equivalent of) 10 pumps, with a 24" barrel.... and 820 fps before it retained air in the valve with those springs (at the equivalent of 16 pumps).... An 18" barrel should lose less than 100 fps from those numbers....

The reason I quote "equivalent pumps" is that the pump assembly was from a 2200, which has a 50% greater swept volume, so the pumps quoted would be for a stock 13XX pump with F-T-P....

Bob

fritz · Silver Status Member

Sorry for the thread jack, but that's good info, as always.

What do you do with the transfer port? It seems like the stock one fits very loose.

I've changed my mind for the sake of wisdom, and will convert it to .22.

When I order parts from crosman, will I need a separate transfer port for .22? I plan to get the backpacker pump arm, carbine stock, barrel, breech (long for when I decide to optic), 22xx barrel band, disco hammer spring, and 13xx sear (as you suggested for the valve, weakened).

Does the discovery barrel band (aluminum) fit the 13xx tubes? I'd like to NOT install plastic if possible when it comes to holding my barrel in place.

I'll contact Mellon for the FTP/V kit, roll pin delete (I like the clean look of his), and long probe bolt.

Top it off with a Tko22 muzzle brake with a front sight, and Williams peep.

rsterne · Moderator

Stock Crosman transfer ports, I find, generally seal up just fine, and can be drilled out large enough for .22 cal.... The optimum, for maximum power, in that caliber, is about 0.161" for the transfer and barrel ports, and the valve exhaust port.... which can also benefit from angling about 20* towards the throat.... Even the throat itself can be successfully enlarged when building a .22 cal pumper.... and in fact boring out the inside of the valve, and shortening the threads, both of which increase the internal volume, can give extra power, at the expense of a bit more pumping.... It isn't that difficult to get about 600 fps at about 13-14 pumps with a 14.5" barrel in .22 cal. with 14.3 gr. pellets....

The Disco barrel band will not fit the 13XX tubes, they are only 3/4" OD, the 22XX and Disco are 7/8" OD....

Bob

fritz · Silver Status Member

Good thing I asked. Unless I read it wrong, Mellon lists the disco band to replace the 22xx, and the 22xx for the 13xx replacement.

Maybe if I got the machinists at work a 6 pack they'd bore a valve for me, but I mostly have to work with off the shelf or aftermarket drop in parts... for at least a few years. Not too much I can really do with just a drill press.

Thanks bob. It's appreciated.