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PostPosted: Sat Jul 28, 2012 9:07 pm 
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Now that I'm starting with a "full power" AT44, I'm moving the thread over to "Mods and Repairs" as that is more appropriate from here on.... Here is the previous thread: topic48744.html

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 6" 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....

Image

Image

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

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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: Mon Jul 30, 2012 5:42 pm 
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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....

Image

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

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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: Mon Jul 30, 2012 11:16 pm 
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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....

Image

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....

Image

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....

Image

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....

Image

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....

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: Thu Aug 02, 2012 12:34 pm 
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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....

Image

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

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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: Thu Aug 02, 2012 2:05 pm 
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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....

Image

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;
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: Thu Aug 02, 2012 3:32 pm 
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I'm finding this post very intriguing. Can you help me understand what the Vel. and Pres. on the chart? I'm thinking the Pres. is the hammer preload?

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PostPosted: Thu Aug 02, 2012 7:08 pm 
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The pressure is the air pressure in the reservoir in bar.... 1 bar = 14.5 psi, so 200 bar is 2900 psi.... Sorry I forgot to change the units on the right axis.... The dots on the lines are the pressure at the start, peak, and end of the string....

NOTE: pressures on the graphs are in bar, not psi....

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: Thu Aug 02, 2012 11:29 pm 
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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....

Image

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;
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: Fri Aug 03, 2012 11:32 pm 
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Well, today was kinda wierd.... I took the rifle all apart and matched up all the ports to smooth and maximize the flow.... I ended up with a 0.180" valve top port, 0.183" transfer port, 0.185" through the breech, and a 0.187" barrel port.... I polished the barrel with some bore paste and stuck everything back together and gained a whopping 6 fps (0.5 FPE) for my trouble.... but at least it didn't use any more air, so the efficiency was a solid 1.20 FPE/CI.... Basically all the ports downstream of the valve are as big and smooth as I can go.... By now I figure the choke point is the valve seat itself.... It looks like this....

Image

There was a spare supplied with the parts that came with the gun, and it measures 0.171" ID, less the area of the 0.100" stem.... so the equivalvent area is the same as a hole measuring 0.139" (the same size as a stock Disco transfer port).... This is obviously a lot smaller than the rest of my ports.... I figured if I ruined the seat I had a backup, so I knocked it out of the valve and measured it.... and found that the hole in it was even smaller than the spare, it only measured 0.163" ID.... I mounted it in the lathe, and using my number drills, one at a time, slowly took it all the way to 0.187".... the same size as the ports (but still less area because of the stem).... The throat area is now the equivalent of a 0.158" hole, 50% larger area than the one I took out.... There is enough meat to drill the hole out to about 0.200", and I can thin the stem down to 0.090", and match the throat area to the area of the rest of my ports if I need to.... However, I know that if I make the valve too big, the hammer may not open it.... so I stopped at 3/16".... stuck the gun back together and shot some more strings....

WOW!.... What a shocker.... I still had the hammer preload at 7 turns out, hoping that it would have enough oomph to knock open the larger valve at 200 bar.... Well, the first shot over the Chrony was 990 fps, then 1010, and it peaked at 1023 fps before falling back to 983 at shot 15.... Drilling out that valve seat gained me 75 fps and 8 FPE.... and the hammer still worked !!! .... I dialed the preload down to 6 turns, and then down to 5, and the gun was still shooting way stronger than before.... I set the preload back to 7 turns out and shot a string with H&N Baracudas and EunJin Domes.... Here are the results....

Image

The dotted black lines on the graph are from the first string today, JSB Kings at 7 turns out, before I drilled out the valve seat.... That was the most power up until today.... The red pressure line can be used for all the pellets at 7 turns preload.... Here is a summary of the results:

25.4 gr. JSB King: 1023 fps (59.0 FPE) max.
1009 fps (57.5 FPE) average for 15 shots @ 1.06 FPE/CI

30.9 gr. H&N Baracuda: 957 fps (62.9 FPE) max.
944 fps (61.1 FPE) average for 15 shots @ 1.13 FPE/CI

35.2 gr. EunJin Domed: 915 fps (65.5 FPE) max.
899 fps (63.2 FPE) average for 15 shots @ 1.17 FPE/CI

That's a 9-10 FPE gain over yesterday.... The shot count is a few short of the 18 (two full clips) that I want.... but my power goal of 50-60 FPE has certainly been achieved.... and by enough margin I am confident that I can get an 18 shot string in the 950 fps range with any pellet from 25-31 gr., and that includes all the ones with a decent BC....

It's been quite a day.... Using JB Weld on the groove in the valve will have to wait, the tubes I have are so old they are hard as a rock.... Tuesday (after the holidays) strangely seems a long way off.... but I'll enjoy the weekend knowing I've accomplished my goal with this rifle....

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: Mon Aug 06, 2012 1:59 am 
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Hi Bob,
I'm trying to decrease speed and increase shot count on a BT65. I like the idea of blocking the holes in the valve. May I ask, how did you get the brass bush out? Also I'm in Australia and don't understand the size tap you used (6-32) what does this size mean? Hope you can help.
Peter


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PostPosted: Mon Aug 06, 2012 9:59 am 
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Not sure what you mean by "brass bush".... The part where I drilled and tapped the holes is the valve itself, I don't know if the BT65 is different.... The valve threads into the end of the tube in the AT44.... The screw size (6-32) means a #6 screw with 32 threads per inch.... The tap just happens to fit in the holes in the AT44 valve, and the setscrews are a common size in North America.... It would just be a matter of using the smallest screw you can fit in your valve.... or using JB Weld epoxy to glue up the holes and the groove.... If you are trying to reduce the velocity, I would make sure one hole lines up with the transfer port (ie "on top" and then fill the other five and the groove, making sure the epoxy isn't inside the cavity in the valve.... If you don't have enough power, you can always drill out the top hole a bit at a time....

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: Mon Aug 06, 2012 11:34 am 
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wow 65 fpe..when i had a bt65 with the much larger valve is was only able to get 67...now im wondering if 80+ fpe is possible with bt65


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PostPosted: Mon Aug 06, 2012 12:27 pm 
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greengoblin wrote:
wow 65 fpe..when i had a bt65 with the much larger valve is was only able to get 67...now im wondering if 80+ fpe is possible with bt65


Quote:
im wondering if 80+ fpe is possible with bt65


As i recall i was up to around 75 ft/lb with my bt 65 ... i can't remember what my at 44 in 25 cal was getting but it was not any were near 65 fpe............... nice work Bob .......................


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PostPosted: Wed Aug 08, 2012 12:08 am 
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Location: Victoria, B.C.
lauchlin wrote:
As i recall i was up to around 75 ft/lb with my bt 65 ... i can't remember what my at 44 in 25 cal was getting but it was not any were near 65 fpe............... nice work Bob .......................


Hmmm...I wonder who has that AT now... :) Nice work Bob, unfortunately I haven't shot mine in awhile, I really should get my air tank filled up someday.


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PostPosted: Wed Aug 08, 2012 3:57 am 
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Sorry Bob,
By brass bush I mean the brass section that the valve screws into in the tube. You said you cut it back to align the ports. In my diagram of the BT65 there appear to be two but I have no idea how to get them out.
Peter


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