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Thread: Hatsan .25 cal AT44S-10 Long Mods

  1. #1
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    Hatsan .25 cal AT44S-10 Long Mods

    Now that I'm starting with a "full power" AT44, I'm moving the thread over to the "PCP" section as that is more appropriate from here on.... Here is the previous thread: http://www.canadianairguns.com/showt...Long-in-25-cal

    The first mod is a relatively simple one that allows the hammer spring preload to be adjusted without removing the stock.... I didn't invent this, just repeating it here for convenience.... The hammer spring preload in the AT44 is adjusted by a screw residing inside the hammer.... It is accessed through a hole just below the safety, using a 4mm allen key about 7" long.... Grinding/filing a slot in the top of the stock allows the allen key to enter the hole.... The hammer is free to rotate, so you also need a way to prevent that.... There is a hole in the bottom of the main tube, 1/2" behind the main stock mounting bolt.... If you peer in through that hole, you will see the hammer, which has a matching hole in it.... The gun must be uncocked for the holes to line up.... Inserting a pin through both holes locks the hammer so you can adjust the preload screw.... Unscrewing the adjustment (ie CCW when viewed from the rear) INCREASES the tension.... Here are photos of the mods to the stock....





    Once the stock is back on the gun, you can insert a small allen key into the hole in the bottom of the stock until it engages in the hole in the hammer to prevent it from turning.... Grinding a taper on the end allows you to slide it in easier if the alignment of the hole in the hammer isn't perfect.... If it won't drop into the hole in the hammer, then rotate the hammer using the long 4mm allen key until it does.... Note that on the pump action, the hole in the tube is further back, and the hammer must be cocked for the locking holes to line up.... The hole in the stock must be drilled further back as well....

    On my rifle, the preload screw moved easily, but I have seen them either loctited or squashed flat to make them tamper resistant.... If the screw rotates too easily, it may be necessary to put a drop of loctite on it once you find the best setting....

    Bob
    Last edited by rsterne; Jul 28 2012 at 11:45 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

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    Here is the data for various hammer spring preloads using JSB King 25.4 gr. pellets in the "full power" version of the AT44S-10 Long.... NOTE: pressures on the graph are in bar, not psi....



    As you can see, increasing the preload from 2 turns out to 4 increased not only the power, but also the shot count by a couple.... going another 2 turns was a bit too much to achieve maximum shot count, but set up that way the rifle delivered 16 shots averaging 46.6 FPE at 0.97 FPE/CI, so backing it down just slightly should produce 2 clips (18 shots) at about 45 FPE with a 200 bar (2900 psi) fill and bump the efficiency to just over 1.00 FPE/CI....

    One small glitch has surfaced, something I knew I would have to watch for from previous experience with a .22 cal AT44.... The hammer spring preload tends to back itself off as you fire the gun, so it will need either some loctite or a locking device on the screw to prevent that.... I'm thinking a hole in the screw with a plastic plug inserted to create friction will probably work....

    Bob
    Last edited by rsterne; Aug 02 2012 at 08:09 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  3. #3
    Senior Member SeanMP's Avatar
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    That's what I have done a couple times for loose adjustment screws. After drilling and tpping a hole I dropped a small snip of 095 weed trimmer in the hole followed by a set screw. That gave a nice adjustable friction bushing

    Has this gun had the valve mod that you described done to it yet?
    Sean

  4. #4
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    I pulled the gun apart tonight and fixed the problem of the hammer preload screw moving.... I drilled a 1/8" hole about half way through and drove in a Delrin pin and then shaved it off flush with the top of the threads and screwed it in.... It was a bit too tight, but a little shaving with a razor blade and now it's nice and snug but you can adjust it when you want to.... Here is a photo of the hammer and the two allen keys to adjust and hold it.... There is a spacer between the screw and the spring, and it and the hammer spring are sitting on the long allen key I made....



    I also pulled out the brass sleeve that the valve screws into and checked to see how it lined up with the holes in the valve.... It was just a fraction off lining up with one of the holes, so I machined 0.001" off the end so that when screwed in, one hole lines up perfectly with the transfer port.... The sleeve barely comes flush with the end of the main tube, so if you had to machine off more (0.010" would index the valve about 1 hole) you might also have to machine a small amount off the end of the main tube as well.... I added a punch mark to identify the top hole in the valve.... I also noted that the annular groove in the valve is narrower than the transfer port where the circle is on the photo....



    Previous experimenting by others on the AT44 showed that plugging the lower three holes has little affect on velocity and reduces air use, thereby increasing efficiency.... I disassembled the valve, tapped the bottom three holes 6-32 (no drilling needed, they are the right size) and installed 3/16" long SHSSs flush with the outer (and inner) surface of the valve.... This not only blocks the holes, but helps reduce the volume of the groove.... If it proves to be the right thing to do, I will fill the remainder of the groove between the screws with JB weld to further reduce the volume....



    I put a slight bevel on the edges of the groove to fair it into the transfer port.... I will likely be drilling out the top hole to the same size as the TP (0.172") once I have tested this stage.... I may also be plugging the remaining two side holes as well.... that proved to be the best, when used in conjuction with the larger top hole in the .22 cal I worked on....



    The next step is to reassamble the rifle and test it with the lower 3 holes plugged to check the velocity and efficiency, and what hammer preload adjustment has to be made to compensate....

    Note to Sean RE the valve threads.... The OD of the threads is 1.080" and the OD of the main tube in that area is 1.202" so the maximum wall thickness above the threads is 0.061" (and it's probably a whisker smaller than that).... The very end of the tube where the O-ring seals is 1.150" ID, so the wall thickness above the O-ring is only 0.026".... I don't even want to think about that.... I guess it works because it's a very short length at that size?....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  5. #5
    Senior Member SeanMP's Avatar
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    Note to Sean RE the valve threads.... The OD of the threads is 1.080" and the OD of the main tube in that area is 1.202" so the maximum wall thickness above the threads is 0.061" (and it's probably a whisker smaller than that).... The very end of the tube where the O-ring seals is 1.150" ID, so the wall thickness above the O-ring is only 0.026".... I don't even want to think about that.... I guess it works because it's a very short length at that size?....
    Your right that would creep me out big time. I did the calculation though at the low end of CrM and your still looking at 4300psi burst. And if the Oring completely fills this reduced area then more than half the force is being taken by the plug. Hehehe I wouldn't do it but theoretically its fine.

    That valve mod makes perfect sense now that I see it in action. With that much void space it would induce those little orifices to instantly hit choked flow because of the sudden pressure drop.
    Sean

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    I got some time to shoot some strings last night to test the plugging of the three lower holes.... I really like my "Chrony Connect" program and cables I got from the UK, it saves me having to stop and record every shot.... The velocities are stored in a file that I can copy into Excel.... I shot three strings with the hammer preload adjusted 6, 7, and 8 turns out from full in.... The new plastic locking plug in the adjusting screw works perfectly, NO sign of any creep in the adjustment any more.... Here are the results.... NOTE: pressures on the graph are in bar, not psi....



    The first thing I noticed was how much smoother the curves are.... They don't have the large shot-to-shot jumps I saw previously.... It requires two turns more preload on the spring to get the same average velocities as with all six holes open.... but the peak velocity occurs at a slightly higher pressure than before, making a more even bell-curve rather than one with a long front end and quick fall-off after the peak.... This is typical of what I expect to see on restricting the flow slightly.... I also achieved an increase in the efficiency, ie slightly less air was used to generate the same FPE.... 7 turns out gave me 23 shots with 4% averaging 898 fps (45.5 FPE) filling to 200 bar and shooting down to 140, with an efficiency of 1.20 FPE/CI.... That's already a pretty potent quarter-bore....

    I'm happy with these results, as they are consistent with what I saw on the .22 cal I worked on.... The bigger caliber required two turns more preload to get back to the same power, whereas the .22 cal only needed one.... I think all we are seeing there is the need for more airflow in the larger caliber.... The next step is to plug the remaining two holes and drill out the top hole to match the transfer port size....

    Bob
    Last edited by rsterne; Aug 02 2012 at 08:09 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

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    This morning I had the opportunity to plug the remaining two side holes and drill and chamfer the top hole.... Here is what the top of the valve looks like now....



    I added some black ink around the hole to make it more visible.... I milled the top hole out to 5/32" and then moved the mill each way 0.006" to enlarge the hole to 0.170" (the same size as the transfer port).... I then plugged the two remaining side holes with 6-32 x 3/16" SHSSs, got out the Dremel with a 1/8" ball grinder tip and smoothed off the front corner of the hole, plus the two side screws where they protruded into the (now larger) top hole.... Once I was finished, I could see the white plastic valve seat, which was "around the corner" before.... The total area of the larger single hole is the same as the three smaller holes from the previous test.... If I gain any performance (or efficiency), it will be from improved flow of the single, streamlined corner and/or the lower drag of a single larger "pipe" compared to three smaller ones.... Assuming this mod works as intended, I will add some JB Weld between the 5 set screws in the groove to get rid of the last bit of wasted volume I can....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

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    Here are the shot strings with the 5 holes plugged and the top hole enlarged to 0.180" and streamlined.... It is obviously flowing a lot better, because at 6 turns out on the hammer spring preload, the velocity is back to what it was with all 6 holes open but the efficiency has increased....



    There are three strings above with 25.4 gr. JSB Kings at 5, 6, and 7 turns preload.... and then two additional strings shot at 7 turns out with 30.9 gr. H&N Baracudas and 35.3 gr. EunJin Domes.... The 7T Pressure line applies to all three pellets shot at 7 turns out....

    A comparison between the shot string at 6T with the JSBs and the first one shot in the "full power" mode with all 6 holes open is very interesting.... The peak velocity (924 fps) and the average (909/911 fps) is nearly identical.... In the original setup, only 17 shots fell within 4% of the maximum, with the first part of the curve "missing".... Now, I'm getting 22 shots, with the curve shifted down about 13 bar at the peak.... The efficiency has increased from 1.03 FPE/CI to 1.21 FPE/CI, so not only am I getting 5 more shots, each shot is using less air.... In fact, I'm only using 6 bar more air for 22 shots than I was originally for 17 shots....

    Here is a comparison for the three pellets at 7 turns preload:

    25.4 gr. JSB King: 944 fps (50.3 FPE) max.
    930 fps (48.8 FPE) average for 17 shots @ 1.19 FPE/CI

    30.9 gr. H&N Baracuda: 882 fps (53.4 FPE) max.
    870 fps (50.9 FPE) average for 20 shots @ 1.24 FPE/CI

    35.3 gr. EunJin Domed: 837 fps (54.9 FPE) max.
    825 fps (53.3 FPE) average for 20 shots @ 1.29 FPE/CI

    In order to get two full clips with the JSBs I would need to back the preload down just a whisker and lose a few fps.... Although I have achieved a peak of over 50 FPE with the JSBs and nearly 55 FPE with the EunJins, I'm not quite done yet.... When I had the breech off, I noticed that the hole in the breech and the barrel are slightly smaller than the transfer port.... I think that needs correcting....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  9. #9
    Senior Member SeanMP's Avatar
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    When I had the breech off, I noticed that the hole in the breech and the barrel are slightly smaller than the transfer port.... I think that needs correcting....
    Lol...of course you do

    I have one request. Do that correction and the epoxying of the dead space in separate steps. I'm really interesting in seeing the effects of that dead volume

    50+ FPE....Astonishing. And simple

    All those fellows that are always kibitzing about wanting more power should take note of this thread. Get your PAL and have att'er. There is no complicated machining done here
    Sean

  10. #10
    Administrator AirGunEric's Avatar
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    It's posts like this that make me buy sh!t and then determine I never have time to actually do the work (I have an AT44 w/extra tank and valvestems collecting dust in a case)...

    Somebody stop Bob before I go broke...
    I'd say I care- but I'd probably be lying...


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