My Benjamin Trail NP rebuild

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Offline djedi

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My Benjamin Trail NP rebuild
« on: July 17, 2012, 05:03:28 PM »
Hello fellow air gun fans!
I've already learned quite a bit from this forum having lurked here for weeks now,  but I did not feel the need to post anything since I had nothing of my own to contribute.  As of yesterday that changed a bit.  Bear with me please.  I know this is a long one.

I own two air rifles.  An old .17 under arm Chinese bargain rifle and a shiny new (relatively) Benjamin Trail NP.  But after a couple months playing and putting about 500 rounds through it, the Benji is still nowhere near as accurate as the old China .17.  I tried the artillery hold, I tried a variety of the best pellets, I tightened all the screws, even changed scopes, but still could not shoot consistently tight circles at 15 yards.
So yesterday I took the plunge and tore the thing down. I'm not a gunsmith, but consider myself rather adept mechanically and electrically, so undaunted, I delved right in.  After reading all I could on these forums I decided on several tasks - replacing the trigger,
replacing the piston seal, cleaning out the old oil, and relubing.  I had already deburred the OEM trigger and pulled out the secondary spring in earlier attempts to increase accuracy, so the pull was actually fairly clean, but if you're going to get into it- might as well dig in up to the elbows.  Building the spring compressor was fairly easy. I took a cue from the excellent pyramid video on rebuilding the Benji and used a similar design. Of course, to complicate matters, I had already switched to an excellent RWS scope which used 30mm rings, so finding the appropriate piece of tubing took a bit of hunting, but 1" galvanized metal electrical conduit happens to be exactly that OD. Glued & tightened into a threaded connector that happens to be close enough to NPT to screw into a black elbow quite firmly, made the connections to a nipple and flange pretty simple. (See photo)

When Combined with a 2X4 and an amputated 5" C-clamp and spacers, this contraption made for a fine compressor.
As recommended by another member here, I purchased JM's moly paste for re-lubing and a shiny new Tesla Custom Piston Seal.  I had files, emery cloth, degreaser, punches, needlenose, and various other implements ready, so I embarked.
The job went well except for a few humorous small hitches.  I will describe them in detail to save some other members possible consternation. Upon disassembly of the rifle a few things became  quite obvious. The original factory seal, though still serviceable, did have some heavy gouges in the leading edge - probably due to hasty and forceful assembly. The inside edges of the punched out openings in the compression tube were indeed sharp and a bit jagged. But these cleaned up easily with a needle file and some emery cloth. I noticed on several videos that the actual piston and gas spring seemed to come apart quite readily. Mine did not. Since I was not in a forceful mood, I decided to work with them as one unit. This actually counted in my favor, as you will see.  After some prodding, the old seal came off, in good shape actually.  I then noticed the strong back-bevel that held it in place on the  piston. I also noticed the same (opposite, of course) bevel on the new seal. Try as I might (without damaging the new seal) I could not get that slippery little sucker on there.  When it jumped halfway across the garage into a pile of sawdust, I decided to do an internet search on the matter. I found that there existed a little device known as a seal installer.
GREAT!
Unfortunately, I did not have one, and I was determined to get this puppy back together today. OK, so I would make one that looked exactly like the unit in the youtube.  I would turn it out of a piece of dowel that I would place in the chuck of my drill press. But a funny thing happened on the way over to the drill press.   I noticed a set of sockets on the shelf, and a few of those babies were tapered down so a 3/8 square wrench would work on a smaller nut.  EUREKA!  I could just barely press the seal onto the smaller end! And the larger end was just a tad larger than the piston dovetail at the top! Using a deep socket that just fit on the outside edge of the seal, a quick tap with a 2 lb hammer fit it onto the piston like a charm! (see photos).


Feeling all proud of my ingenuity, I lubed the piston, seal, gas ram combo and started it into the compression tube.  It was tight.  Very tight.  Much tighter than the old seal.  So tight, in fact, that I had to use a small plastic hammer, carefully pushing on the seal with a small piece of rounded wood to keep it from getting chewed in the stamped openings. It was tough going, but I persevered, figuring this tightness would give it more force and better compression. I replaced the trigger with the new golden model, playing with the adjustment screws just a bit, and then started to put the whole gun back together. But just before reassembly I noticed that where the breech seal o-ring contacted the compression tube, the surface was almost wavy, and had a small gouge out of the area where the seal made contact.  Another jig had to be created (since I did not want to force out the breech locking pin) to keep a small smooth file perfectly flat, while I cleaned up that area without removing enough material to compromise the air seal.  After a bit polishing that filed area I reattached the barrel assembly and cleaned up the exit hole. 
Excited, I went outside, aimed at a target, pulled the trigger, and then...........
Pup.....pftttttttt,   pfffffft, and nothing.
Opened the breech, saw the .22 pellet still there, closed it up, figuring maybe it has to "work itself in" and tried it again.
Pup.....pftttttttt,   pfffffft, and nothing.
Did that 4 or 5 times - Just absolutely in awe of the gun's negative reaction to my careful and loving tune-up.
More research on the internet followed. Finally I saw one posting by someone apparently knowledgeable that Chinese tubes may not always be consistent, and that these seals may have to be "fitted". 
GREAT!  How do you resize something with the approximate consistency of a gummy-bear?  More research.  Apparently, these things CAN be sanded down carefully!  But how would I  make it even on all sides?  First thought that came to my mind was a lathe. But I was not looking forward to pulling that jumpy rubber ring off the front of that piston. Looking at the thing carefully, I noticed the gas spring had a small threaded hole on the end closest to the trigger.  Remember- my gas spring did not  seem to be easily removed from the compression tube! This fact actually came in handy here.  I was able to force an 8-32 screw into the end of that piston. I cut off the head and placed that into my drill press chuck after turning the table aside. But the whole thing seemed too wobbly to even test without something holding the lower end to keep it from flailing all around. So I drilled an oversized hole in a 2x4 the size of the piston and clamped that to the table. Now, once greased, the assembly could turn easily and not wobble. I then grabbed various grits of emery, including some plumber's cloth (which has an open weave).  This worked fabulously, and in no time I had taken the little bugger down so the end facing the piston was a hair smaller in diameter than the piston, but the front still flared out beyond it. After two attempts to reinsert the seal I got it to where it could slide without undue force being necessary.  As the "expert" poster had suggested-  where it would require about 2 lbs of pressure to move it. (I'm sorry I did not take a photo of the piston in the drill press- it was actually quite humorous, but I was too caught up in the process at the time).  Reassembled, I took the gun back out and aimed it at the target. 
POP!    What a sweet sound!
And that was the pellet I could not eject on my previous attempt before re-sizing the seal! Since then I've run about 50 pellets through her. She seems improved, but not quite as much as I was hoping.  I could get pretty good groupings (2") at 35 yards with CPHP's, but I'm hoping things will improve when she settles in and I test her with Kodiaks or RWS Superdomes or Beeman FTS's.  Thanks for reading my lengthy post, and I would welcome comments, suggestions, and further banter on the subject.
Wolfgang
« Last Edit: July 17, 2012, 06:28:53 PM by djedi »
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Offline robert w

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Re: My Benjamin Trail NP rebuild
« Reply #1 on: July 17, 2012, 07:28:24 PM »
great post and might take it a tin or 2 to break it in. id shoot it a bunch before discounting it has another glitch . i always figure the first 2 tins of pellets are for breakin it in then its time to get on with accuracy .
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Offline PaperPunch1

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Re: My Benjamin Trail NP rebuild
« Reply #2 on: July 17, 2012, 10:49:23 PM »
Mine likes the H&N Field Target Trophy 14.66gr domes. CPHP won't shoot consistant, nor will many others I tested. The thing that finally settled mine down was the addition of weight to the last several inches of the muzzle, in the form of a bag of #9 lead shot from a shotshell....about 4 of them. Anyway, I also drilled a hole inside the stock and added shot inside too. This makes for a heavier rifle, but the accuracy improved. I believe that the weight at the muzzle, simply lessoned the amount of movement "before the pellet left the barrel".
I even thought of shortening the barrel, but don't need to now.
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Offline fdisker

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Re: My Benjamin Trail NP rebuild
« Reply #3 on: July 17, 2012, 11:26:00 PM »
As paperpunch said, keep trying different pellets.  I had several "eureka" moments recently when trying to get my nitro piston Benji Titan to group.  Took it completely apart for a lube tune and was extra careful not to tear the new seal during reinstallation.  So proud of myself after getting the rifle back together and feeling the super smooth action of a newly lubed nitro piston!

Then, disappointment.  The tuned gun shot no better than before.  Anger, depression, sadness .... none of those really, just frustration because I thought I'd figured it out.  Only after finding the right pellet (H&N FTT, just like paperpunch) did everything come together.  Nice, tight groups out to 80 yards.

Will they group that well in your Trail?  Maybe, maybe not.  The point is you need to keep searching until you find the one that works best.  Keep searching.  You'll get there.
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Offline djedi

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Re: My Benjamin Trail NP rebuild
« Reply #4 on: July 18, 2012, 10:26:40 AM »
Thanks guys.  Shot another 50 or so rounds last night.  Beeman FTS coppers.   Huge difference when I took the first shot. Way high, then the next 2 flew all over the place (6 inches apart at 35 yards).  BUT I then remembered that I played with the barrel shroud after shooting the night of the tune.  I did not wrench tighten it- only hand tight.
So I took the shroud back off and checked the barrel for trueness.  It was perfect when I put my metal straightedge on the side, but vertically, it almost had an "S" curve in it! Wasn't too far out, maybe half a millimeter both ways, but definitely out.
MANURE!   :'(
I put the shroud back on, but this time pretty tight.  Consistency improved quite a bit.  Especially with a very loose artillery hold.
My questions follow:
Anybody know how much a slightly out-of-true barrel will affect consistency?
How tightly should that shroud be fixed on the Benji?  Does the shroud really help true the barrel, as it appeared to on mine?
Will it be worth my while building a barrel straightener or could I just mount 2 pieces of 2X4 on a base, say 8" apart and sharply tap the barrel in the opposite direction of the bow (using thick pieces of cowhide as padding) ?
Or, should I just get another barrel and hope that one will be straight?
Thanks in advance for answers.
Wolfgang
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Offline Muppit

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Re: My Benjamin Trail NP rebuild
« Reply #5 on: July 18, 2012, 11:45:45 AM »
barrel is under 20 dollers from crosman but some times take a little work to get to fit right . with that said if i under stand you correctly your barrel has a wave to it, is that right? that may be hard to straighten. i have used a hammer and wood to straighten barrels before(i'm not always a patient person, not good at times) but these were bent at the breech block only and cuased by lossing control of barrel while cocking. i cant see that the shroud would help straighten the barrel any it slides on, on one end and threads on the muzzle, so the barrel could be bent inside the shroud......dave
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Offline djedi

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Re: My Benjamin Trail NP rebuild - dave
« Reply #6 on: July 18, 2012, 12:29:20 PM »
Thanks Dave, but as I see it, the threads are at the business end of the barrel and the shroud sits tight against the breech block at the near end.  Seems the harder you screw that shroud down, the harder it pushes on the block, in effect, exerting outward pressure on the crown end, and, in effect, pulling it tight, if not even a bit straighter.  Please tell me if my logic is incorrect.
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Offline Muppit

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Re: My Benjamin Trail NP rebuild
« Reply #7 on: July 18, 2012, 03:26:17 PM »
that makes sense but the shroud is made of thiner material and i thing whould stretch or bow before the barrel would. who knows for sure
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Offline Berky

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Re: My Benjamin Trail NP rebuild
« Reply #8 on: July 18, 2012, 06:09:04 PM »
Maybe I mis-understood something here or just missed something. No to be a smart ^&* but why do you need a spring compressor for a NP gun?
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Offline Muppit

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Re: My Benjamin Trail NP rebuild
« Reply #9 on: July 18, 2012, 06:26:15 PM »
it tough to get the pin in the rear block without one. jes it can be done because the block only sticks out a little but with a good ram its still hard to get the pin in........dave
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Offline Berky

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Re: My Benjamin Trail NP rebuild
« Reply #10 on: July 18, 2012, 06:44:48 PM »
Are these built differant than a NP XL?
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Offline Muppit

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Re: My Benjamin Trail NP rebuild
« Reply #11 on: July 18, 2012, 07:14:44 PM »
same priniple differant part numbers the xl has a larger compression chamber to start with so most everything will be slightly larger
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Offline Berky

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Re: My Benjamin Trail NP rebuild
« Reply #12 on: July 18, 2012, 08:35:34 PM »
I still dont understand.  :o   I have a NP XL and didnt need a spring compressor to work on it. Help me understand why you need one for a NP model. Thanks
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Offline djedi

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Re: My Benjamin Trail NP rebuild
« Reply #13 on: July 18, 2012, 10:58:17 PM »
You are correct  and incorrect, Berky.  The Trail np XL does NOT need a spring compressor to disassemble.  The Standard NP DOES however.  Who knows why the difference in designs exists?  My guess- the manufacturer did not want to use an extra heavy piston in the standard np for cost reasons, but still wanted to get high fps numbers, so they "pushed" things a bit by adding a longer back spring guide (even though there is no spring) which pushes the whole mechanism tight against the np's locking pin. When pressure is released by compressor, you have about 1/2" sticking out the compression tube.
Muppit:  A well designed tube pushing down like that can indeed exert quite a bit of pressure.  Perhaps not enough to straighten the barrel, but certainly enough to reduce vibrations in the barrel.  I just wonder if straightening the small curves would have any effect on accuracy, that's all.
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Offline john

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Re: My Benjamin Trail NP rebuild
« Reply #14 on: July 19, 2012, 12:17:51 AM »
Great post including some crucial info rarely mentioned in other tuning descriptions (e.g. 2lbs force to seat a properly sealed piston into the compression chamber, not into the compression tube because the piston/seal combo can be much tighter in the slotted part of the tube than in the compression chamber forward of the cocking slot). I don't quite understand your use of the socket. Regarding bowed NP barrels, I have two....however, a bow in the barrel does not necessarily mean a bow in the bore...nor does a straight barrel guarantee a straight bore. But the magnitude of these bends is too small to be important imo; what IS important is whether the bend is downward causing so much barrel droop as to be uncorrectable with scope or mounts. Regarding shroud, if tightening improves accuracy, it will be due to improved harmonics, not due to straightening of the barrel imo. Mine likes Superdomes fwiw.

The Trail np XL does NOT need a spring compressor to disassemble.  The Standard NP DOES however.
With respect, I believe you are exactly 180 out on that. Just the opposite. The NP has only about 1/2inch preload on the gas ram so, if the barrel is removed from the compression tube, a long clamp can be used (all tools must be used intelligently of course). The XL has enough more preload to make a spring compressor advisable.
« Last Edit: July 19, 2012, 01:12:56 AM by john »
Benj Trail NP .22 HW, modified shroud, semi-floated Summit stock, home-tuned including deburing, honing, new Crosman piston seal, JM piston buttons, muzzle crowned, trigger group shimmed/debured, padded cocking lever, w/ CP3-9x40AO (3500CPHPs, 500CPUMs, 175CP pointed hunting, 4000RWS superdomes, 250 JSBExpress, 1000 JSBExacts, 250 H&N FTTs, for a total of almost 9K shots so far)
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Re: My Benjamin Trail NP rebuild
« Reply #15 on: July 19, 2012, 11:08:24 AM »
Can someone explain the different way of spelling different? :)

Offline djedi

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Re: My Benjamin Trail NP rebuild -john
« Reply #16 on: July 19, 2012, 04:46:02 PM »
John- Interesting.   You are correct about the 1/2" of compression on the standard np (as I stated) but I have read in several different posts on different sites that the TrailNP XL does NOT require a spring compressor.   Where did you get your information?  Do you own one and have you disassembled it?  I probably could have used a long clamp to release it, but I found the gun much easier to work on when scope-ringed onto my device.
Anyway, here's a link to a post on this forum that seems to prove otherwise:
http://www.gatewaytoairguns.org/GTA/index.php?topic=27565.0
Ho yeah, about that socket.   The fact that I could just barely get the seal on the narrow end of a greased socket, meant that I could now give it a whack (with a large diameter socket) from above and force it to slip down over the wide end of the socket,  expanding the seal enough to fit onto the piston's reverse bevel end without my damaging the seal.
Wolfgang
« Last Edit: July 19, 2012, 04:58:58 PM by djedi »
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Offline Berky

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Re: My Benjamin Trail NP rebuild
« Reply #17 on: July 19, 2012, 05:24:57 PM »
Ive taken my NP XL apart several times and the tube cover has hardly no preload on it whats so ever. The need for a spring compressor on a NP XL is ridiculous!
« Last Edit: July 19, 2012, 05:27:43 PM by Berky »
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