GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: nervoustrigger on April 05, 2015, 11:36:53 AM
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Hey guys,
I was recently setting up a 1270T for my buddy, Andrew, to use for field target and light hunting/pesting duties. The goal was to get just under 12fpe and as many shots as possible, hopefully 30. The bstaley O-ring buffer is the common way to achieve this result and although this type of tune has been done a number of times, there are a number of variables that can be manipulated so I figured I'd document it here for anyone interested.
I started by enlarging the transfer port to 0.094", then added three O-rings (-113 in 70A durometer) and managed to get a string of 26 shots inside of 4%. I then decided to try adding flat washers between the O-rings which is a technique that others have used. After playing around with the stroke adjustment and hammer spring preload, I ended up with a decent tune yielding 34 shots.
A summary of the settings:
transfer port: 0.094"
Bstaley stack: 3x -113 O-rings (70A durometer) and 2x 0.054" thick washers (alternating O-ring and washers)
stock hammer spring
stroke: 2.0 turns from full CCW
hammer spring preload: 7.0 turns from full CCW
Attached is a graph of the shot string and a couple of groups...not my best shooting and I didn't test for the best pellets but I think it's shooting pretty well.
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Interesting that you put the washers between the O-rings. Your two washers equal the thickness of another o-ring.
And you got the results you were looking for. :)
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Thanks for posting this. Very helpful as I would like to get my 1720p up closer to 12fpe also.
Does Crosman offer a transfer port in the 0.094" size? I am not confident in my abilities to drill to that size myself. Also and further detail on the washers used would be appreciated.
In any case, I will try and follow your specs in tuning mine.
Thanks
R
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Interesting that you put the washers between the O-rings. Your two washers equal the thickness of another o-ring.
Hi John, it appears the shot-to-shot consistency improved with the addition of the flat washers. The strings without the washers looked more jagged. I have a theory...when the O-rings are butted against each other, the contact patch between them is very small and the effective height of the stack may vary if they shift slightly relative to each other. Placing a washer between them gives them a flat surface to bear against, keeping the stack more consistent.
Again, just a theory! I would need to run a controlled experiment to confirm.
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Does Crosman offer a transfer port in the 0.094" size? I am not confident in my abilities to drill to that size myself. Also and further detail on the washers used would be appreciated.
Hi Em, according to the information I found, Crosman supplies a 0.043" (red) and 0.065" (silver) transfer port with the 1720T. The one I was working with had the 0.065" and it is aluminum so it is super easy to drill out with a set of numbered drill bits. I just clamped it in my vise with wood jaws so as not to mar it, and drilled it with increasing sizes until hitting 0.094".
If you can't find a way to get it done, PM me about sending one of the transfer ports and I'll do it for free.
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I'd agree with your theory regarding the washers between the o-rings, Nice work there.
Bill
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Does Crosman offer a transfer port in the 0.094" size? I am not confident in my abilities to drill to that size myself. Also and further detail on the washers used would be appreciated.
Hi Em, according to the information I found, Crosman supplies a 0.043" (red) and 0.065" (silver) transfer port with the 1720T. The one I was working with had the 0.065" and it is aluminum so it is super easy to drill out with a set of numbered drill bits. I just clamped it in my vise with wood jaws so as not to mar it, and drilled it with increasing sizes until hitting 0.094".
If you can't find a way to get it done, PM me about sending one of the transfer ports and I'll do it for free.
Got it, thanks.
I have an extra silver TP and will give it a go myself.
0.094" is only about 2.4mm. Do you recall the drill bit size used? Hope I have one that small.
R
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You can use a #42 or a 3/32" bit.
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Interesting that you put the washers between the O-rings. Your two washers equal the thickness of another o-ring.
Hi John, it appears the shot-to-shot consistency improved with the addition of the flat washers. The strings without the washers looked more jagged. I have a theory...when the O-rings are butted against each other, the contact patch between them is very small and the effective height of the stack may vary if they shift slightly relative to each other. Placing a washer between them gives them a flat surface to bear against, keeping the stack more consistent.
Again, just a theory! I would need to run a controlled experiment to confirm.
That sounds like a plausible theory, but the jaggedness I see in your graph is probably close to what I typically see. When only using the O-rings, did you have any type of grease on them to hold in place?
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I did not apply any grease to the O-rings. They seemed to hold snug to the walls. The hammer and inside walls are lubricated with graphite powder. I try to keep viscous materials out of that area.
But to be clear, the graph I posted was the improved, less jagged result with the flat washers. The most variation between any two adjacent shots inside the 4% window is 8fps. Before the flat washers, there were multiples that varied by 12fps. Again, I don't know that credit goes to the washers because of course I had to tweak both hammer spring preload and stroke after their addition to get it back to the 12fpe level.
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Interesting that you put the washers between the O-rings. Your two washers equal the thickness of another o-ring.
Hi John, it appears the shot-to-shot consistency improved with the addition of the flat washers. The strings without the washers looked more jagged. I have a theory...when the O-rings are butted against each other, the contact patch between them is very small and the effective height of the stack may vary if they shift slightly relative to each other. Placing a washer between them gives them a flat surface to bear against, keeping the stack more consistent.
Again, just a theory! I would need to run a controlled experiment to confirm.
I did the same thing with my syn rod. I just used the washer to take place of one oring. Before the washers I only got 41 good shots , after the removing one oring & installing a 2 washers I got it up to 56 good shots. My thought was that with less oring & the washers made a more solid stop for the hammer, meaning less give on the orings creating a more consent stop for the hammer.
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I played with orings in my uptuned Crosman Challenger. I used a .080 port and various configurations of 70D and 90D orings but couldn't get anything but a descending string. So I gave up on the orings and went with an Altaros regulator set at 1,600 psi.
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http://www.gatewaytoairguns.org/GTA/index.php?topic=45450.msg590077#msg590077 (http://www.gatewaytoairguns.org/GTA/index.php?topic=45450.msg590077#msg590077)
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Well that's cheating ;D
No doubt, regulated is easy to tune. That's what I am accustomed to as well. It takes far more trial and error and chrony time to get good results from an unregulated rifle. Had it not been for the generous sharing of knowledge by others that gave me a close starting point, it would have been more time consuming I'm sure.
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I did not apply any grease to the O-rings. They seemed to hold snug to the walls. The hammer and inside walls are lubricated with graphite powder. I try to keep viscous materials out of that area.
If the O-rings are dry, they can move on hammer rebound or as simple as a cocked gun pointed toward the sky. I use moly grease to hold in place, and if too much got in the tube, a quick wipe down of the tube once the o-rings are in place.
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Great thread Jason,thanks for all the info..
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Bumping this old thread to ask a couple questions about this specific set of mods.
1) How hard is it to do these mods? (in terms of hours)
2) does the manual have a good exploded view that will make obvious (along with this thread) what to do?
3) Where do I get those very specific parts? I could probably find the drill bit on ebay. (I only have fractional inch bits) but how about the O-rings and washers?
4) How much louder is the 1720T than a .177 Marauder? How much more accurate is it?
5) You mentioned # of turns from full CCW. Doesn't full CCW make the screws fall out?
6) Where does the Bstaley stack go?
7) Is the Bstaley stack sold as a kit, or assembled from parts bought elsewhere?
Sorry for all the dumb questions; I've never modded a PCP gun before.
Thanks!
I modded my 1377 myself, with the flat top piston and power adjust mods, after finding some good instructions online.
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Bumping this old thread to ask a couple questions about this specific set of mods.
1) How hard is it to do these mods? (in terms of hours)
1-2 hours going very slow for the first time. Tuning may take longer
2) does the manual have a good exploded view that will make obvious (along with this thread) what to do?
Not in the manual, but on crosman website has parts schematic.
3) Where do I get those very specific parts? I could probably find the drill bit on ebay. (I only have fractional inch bits) but how about the O-rings and washers?
Buy a bag of 100 of #113 70D buna-N O-rings. They are also used to rebuild your gun. The o-ring store or many other sources.
Fractional drill 7/64" will do it.
If going with a 15fpe tune, sub the two thin SS washers with a neoprene fender washer 3/4" OD, 0.070" thick, found at local H/W store. No need to grind to fit.
4) How much louder is the 1720T than a .177 Marauder? How much more accurate is it?
Stock form and power, probably close. At 15fpe, you might add a LDC.
5) You mentioned # of turns from full CCW. Doesn't full CCW make the screws fall out?
No. Full CW does.
6) Where does the Bstaley stack go?
Between hammer and back of valve.
7) Is the Bstaley stack sold as a kit, or assembled from parts bought elsewhere?
Just buy a bag of the O-rings above and this spring: http://www.mcmaster.com/#9434k65/=14kjgy5 (http://www.mcmaster.com/#9434k65/=14kjgy5) They also sell o-rings
Sorry for all the dumb questions; I've never modded a PCP gun before.
Search for "marauder pistol assembly". There is a good tutorial out there somewhere.
Thanks!
I modded my 1377 myself, with the flat top piston and power adjust mods, after finding some good instructions online.
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hello here..
smaug2, the ccw rotation of the stroke adjustment is "as viewed from the end where you insert the hex key" i think.
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i got a 1720t early in the year also for field target in the under 12 ft lb cat. In stock form, it was about 9 ft lbs. Only mod I did was to drill the transfer port to .09 using a 3/32 drill bit. Really easy to do. I rotated the hammer spring tension to full counter clockwise. Then chronied from there. At 2 turns clockwise, I was getting 11.76 ft lbs - 710 fps using 10.5 gr pellett. If you want to go 14 ft lbs, you can turn another 2 turns to reach about 780 ft lbs
I've also plumbed the gun to a regulated 22 ci bottle with pressure output of 2100 psi.
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Thanks John, David and Jay!
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What is the stock LW .177 barrel port ID?
Thx Randy
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I used four o-rings and a standard (not drilled out) PRod TP
Using a 7.33 grain JSB Exact, these are my results:
(http://i1356.photobucket.com/albums/q733/barnydaddy/eb8aee7929c23681c81a278dd8e6fdfa_zpspjkpf2aj.jpg)
This was a 50 foot rifle GTA target earlier this year.
I was the only one using a pistol:
(http://i1356.photobucket.com/albums/q733/barnydaddy/3db1e0404824ef5aeb09586a86db5e9a_zps7oplkjqc.jpg)
Here is the item in question:
(http://i1356.photobucket.com/albums/q733/barnydaddy/055620bf35879fa1851804708b1b8dee_zpsuxurewme.jpg)
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The square o-rings do not move in the tube, no need to put anything on them.
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What is the stock LW .177 barrel port ID?
The one I have off right now, measures 0.130", but I might have opened it up in the past.
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I think the pistol comes with the green port installed at .07. There;s a red one in the box at .05. I drilled oversize the red one to .09.
At .09, 10.8 gr is 710 fps, 8.3 gr is 780 fps.