Brand new 1377! Let's turn it into a 100 yard 1377!

[rsterne]

I don't think anyone really has a good idea what maximum pressure you can achieve with a 1377 pump and F-T-P.... I can tell you that a 1377 with stock valve, hammer and springs will start retaining air at about 1500 psi.... It wouldn't surprise me if you could achieve twice that with a good pump, minimum headspace, and enough pumping.... What does that mean?....

The area of the valve (and piston, both 0.62" diam) is 0.302 sq.in.... That works out to 452 lbs. force at 1500 psi and twice that (904 lbs.) at 3000.... The main pivot pin is 3/16", so a solid steel pin (using tensile of 53K, and two ends) would have a predicted shear failure of 1765 lbs (ie a safety factor of nearly 2:1 at 3000 psi).... However, the holes in the tube would be expected to yield (ie start to elongate) at 1075 lbs (safety factor at 3000 psi is only 1.2:1, not really high enough, but should be fine at 1500).... The tube wouldn't be expected to yield until over 9000 psi (11K burst), so not an issue.... The other item of concern is the stake that locates the valve, along with the front trigger screw.... The stake is only 1/4" wide by 0.065" thick, so the area is only 0.0163 sq.in.... Using a yield of 40K for the aluminum valve, that is only 650 lbs. for the stake, but we can add the 8-32 trigger screw, which has a shear strength of 390 lbs. (assuming mild steel with a tensile of 53K), giving a total of 1040 lbs (safety factor of only 1.15:1 at 3000 psi, not really good enough, but again should be fine at 1500).... If you use a high tensile screw (eg. 180K), that would be much higher (1325 lbs. just for the screw) and that would be a very good idea if you plan on pumping a lot.... With that, plus the stake, you would be at 1975 lbs. (about 2.2:1 safety factor at 3000 psi), which should be OK....

So, providing you replace the front trigger screw with a high tensile one, and use a solid steel main pivot pin (I don't have the specs on the stock roll pin, and besides, they wear out the hole in the pump arm quickly), it looks like the first thing to fail would be the steel tube stretching/distorting around the main pivot pin, in theory at just over 3000 psi.... or the pump linkage, which could easily kink, especially if it gets out of line at all.... That is consistent with what I have seen in my Uber-Pumpers.... The first sign of pressure related problems is the back of the aluminum valve wearing/denting where it bears on the stake in the tube.... I would recommend you NEVER pump the gun more than a few pumps without the front trigger screw locating the valve, it's a large part of the safety of the design.... and replacing it with a high tensile 8-32 SHCS is a very good idea, and necessary if you want to pump over ~1500 psi.... The next sign of weakness is distortion of the tube around the main pivot pin.... and I've never seen that, which leads me to believe I have never exceeded 3000 psi.... By the time you get to those pressure levels, the holes in the linkage are going to start wearing and stretching anyway.... It certainly wouldn't hurt to increase the diameter of the main pivot pin to 7/32" to increase the bearing area in the tube a bit....

DISCLAIMER:  This information bears no guarantee of safety at any given number of pumps.... It is simply a bunch of numbers representing my opinion.... If you pump past 10 times, which is Crosman's recommendation, or even just modify the piston, you're on your own....

Bob

[eicodle]

The sky is (nearly) the limit with that platform, as far as modding is concerned.... A Flat-Topped Piston and Valve eliminates the headspace, increasing the compression ratio and allow higher pressures to be reached on fewer pumps.... I have a gun I call my "Uber-Pumper", and it uses a 1377 valve, only slightly modified, with a 24" .177 barrel and a Crosman steel breech.... While the gun could be built on a 1377 pump tube, I chose to modify a 2100 tube and linkage for the job, and make my own flat-toped piston, because the swept volume is 50% greater, so that cuts the number of pumps by a third (ie 10 pumps on my gun is like 15 on a 1377 pump).... If you are using over about 20 pumps and an F-T-P you need the solid steel pivot pin, as the stock roll-pin will wear the hole in the linkage pretty quickly....



With 7.8 gr. JSB Express pellets, I get 500 fps on 3 pumps, 800 on 10 pumps, 940 fps on 20 pumps, and 990 fps on 30 pumps.... It would take about 50 pumps with a 1377 pump and a good, properly adjusted F-T-P to do the same.... but it could be done.... This gun was extensively modified, and isn't just assembled from bolt-on parts, a lot of them I had to make or modify.... but if anyone tells you you can't get significantly greater performance from a 1377 over 10 pumps, they haven't really tried.... It can be done, and done safely, although the wear factor will increase the more you pump.... Even so, I have many tins of pellets through that gun on the original pump O-ring, in fact with no maintenance except the occasional drop of oil.... The keys to higher velocity are a longer barrel, an F-T-P, and more pumping.... All the other things are details....

Before you start on the quest for mega-power, you really need to ask yourself "how many times do I REALLY want to pump this thing?".... Personally, for 99% of people, I wouldn't even change out the hammer spring, as the stock one, without an RVA, is good for at least 15 pumps in a 1377.... With a 24" barrel and properly adjusted F-T-P, that will put you well into the 700's in .177 with a standard weight pellet, without any additional mods.... Beyond that is fun for bragging rights, but you will quickly tire of pumping that much, or at least I did....

Bob

Bob, I'm in the process of making a walnut version of the 1399 stock for my Benji HB22 and I'm interested in the mod you did to your stock.  It looks like you shortened and raised it up a bit?  I don't have a lot of experience with different stock geometries and at this point I'm just scaling down a 1399 pattern a bit to fit the HB22 grips.  My pattern's overall length is about 13.5" now instead of the original 15".  You probably had some good reasons to change the geometry and I'm wondering if I should do the same to my walnut version.

Would it be possible to take a photo of just your modified stock next to a ruler so I can scale things properly?


Thanks,
Eric

[rsterne]

The length was shortened about 1.25", so the LOP is pretty normal (~14.25").... Crosman's skeleton stock LOP is ridculously long at about 15.5".... You can see how far I raised the comb, about 3/4".... helps get your eye closer to the scope C/L....

HTHs....

Bob

[sandman]

By the way, does installing a flat top piston and valve increase pumping effort?

[rsterne]

Not at low pump numbers, but at high pump numbers, yes, because the pumping effort is proportional to the air pressure in the valve....

Bob