GTA
Airguns by Make and Model => Benjamin Airguns => Topic started by: Norm_m on December 08, 2010, 09:02:15 PM
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My fears have come true! For the last several days I have seen some smoke and was able to smell oil. So I had a feeling I had a seal that was beginning to fail. We today I went to the range and my shoots were all over the place no grouping at all! I had been getting dime size groups but not now. I did a Chrony text and got the following:
Test one (my original test) test 2 (today's test)
1- 701.5 1- 575.4
2- 721.9 2- 567.9
3- 717.2 3- 573.8
4- 712.9 4- 558.7
5- 718.1 5- 559.7
6- 721.8 6- 564.7
7- 721.2 7- 553.1
8- 717.8 8- 553.1
9- 718.3 9- 547.9
10-708.5 10- 572.2
high 721.8 high 575.4
low 701.5 low 547.9
avg. 715.92 avg. 562.88
ex. dev. 20.3 ex. dev. 27.5
avg. dev 4.97 avg. dev 7.92
Seeing this NPSS is less than a month old I guess I need to call Crossman. I do not want to send this one back as it has about 1700 rounds fired through it and it is pretty well broken in, so it may be interesting to see what they can do for me. Also I no longer have the original stock as I have replaced it with a walnut one.
Am I correct in saying the seal is gone? How difficult is it to replace a seal and hone out the cylinder?
Any ideas, suggestions or ideas would be greatly appreciated.
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Looks like either a bad seal or bad nitro pistion. Call Crosman & send it back.
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Smoking with a loss of acuracy, I'd say bad seal.
Try the search function for, "Under the hood". Gene did a very good write up of what he did to his NPSS.
Paul.
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a graph of your two shot strings for comparison.
http://img703.imageshack.us/img703/1810/npss.jpg (http://img703.imageshack.us/img703/1810/npss.jpg)
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Called Crossman this morning and talked to Kim. She said they would send out a new piston and to see if that solves the problem. We'll see what happens!
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You may want to also have a new seal on hand when you open it up. A piston that's lost some pressure doesn't account for the smoke, loss of accuracy, and an increase in pellet speed spread.
Paul
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Well I am positive that you will get a replacement seal from Crosman or a piston with new seal on it. Piston alone will not cause the problem you speak of. Kim is one of the leading ladies in parts support. She knows what she is talking about. After all she gets hundreds of calls a day...:) Very smart lady and she has helped me out tremendously over time.
Thank God for Crosman's Support Team..
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Are you getting a replacement piston or a new gas ram?
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Oh duh !
Thanks for the 'wake up' question Perry.
I just realized he said piston and I wrote the same while thinking ram ?
I gotta stop buying cheap coffee !!!
Paul
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Well after knowing the parts were on the way I decided to take things apart to see what makes this thing work. It was much easier than what I expected it to be. So what I have found is that the piston seal has a groove worn it it and I can not see anything that could have caused! I also noticed on one side of the gas ram the finish has been rub of in an area about 1 to 1 1/2 inches. Almost looks like it had been polished in that area. I do have a piston, piston seal and gas ram on the way.
OK so know with the items on the way I want to be ready so what do I need for grease? Is there anything else I may need?
The thing that puzzles me is what caused the grove in the side of the piston seal. It is about as wide as a pencil lead and goes all the way from the front of the seal to the back of it. It almost looks like it had been rubbing on the assembly lever or the beartrap lever. Any ideas on this?
Thanks to everyone that has and will respond to my situation.
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How far back does the piston travel? Does it go back far enough for the seal to reach the area were the slot is in the tube for the cocking lever? I'm trying to see where and how the seal was damaged and it does not seem like it would go back far enough for the cocking levers to touch it. The rest of the tube forward from that opening seems fine
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The seal was probably damaged during assembly and is quite common. The shiny surface is caused by the piston rubbing on the ram because of not centering well. Gene makes a centering kit for some of the guns to resolve the problem but not sure if he makes one for that gun or not. His kits do make a big difference.
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Take a look at the pictures of the damaged seal. This is large defect as it measures .036 deep and .077 wide! Could someone really have some have actually put this gun together and not have noticed or felt that damage being done?
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That is not a defect but could have been caused by a defect. That damage there was for the most part caused by detonation and burned through. It would have been caused by lube being allowed to get in front of the seal which may have been caused by a cut seal during assembly. I've seen many far worse than that.
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Still waiting for my parts to be shipped! I've cleaned everything up, sanded out all rough spots and honed the combustion chamber. All I have left to do is ably the silicone grease & moly paste and install the new parts. Hope to have everything back together by the weekend.
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Yikes, that`s a pretty big cut on that seal.
A trick I`ve read (I believe over here at GTA) when pushing the piston with the new seal back in, is to cut a strip of metal from a soda can and slip that in-between the seal and the interior of the receiver (At the opening for the cocking arm shoe (Very sharp there)), and slide everything inside and pull the soda can metal back out. That way there will be less chance of getting the seal cut again.
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Great minds work alike! I was thinking of trying something like that as that is where damage to the seal could happen. The soda can is a great idea, I was trying to think of something very thin and that is the answer.
Thanks
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Frustration is setting in, put patience is a virtue! The warranty items and another small spare part order from Crossman were completed on Dec. 10 and a week latter it still has not shipped. I have called twice to check on what was going and Spoke to Kim two days ago, she said the shipping department was working on a very large overseas order and she would see what she could do for me. As of 12/17 7AM it still has not shipped! I have the Silicone grease and the Moly paste will arrive today and the NPSS is sitting on my bench all cleaned and prepped for the new seal and piston
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Yes, Dave (ezman604) did a very good write-up about replacing the seal in his Trail using the soda can "shoe horn" sleeve trick.
Just be carefull to make sure the piece of soda can doesn't have any sharp edges the seal can touch.
A tip,
If you cut a few tabs into one end of the soda can sleeve and bend them outward, the tabs will hook on the end of the compression tube. That will free up one hand not having to hold the sleeve in place to prevent it from getting pushed into the bore along with the seal/piston.
Paul.
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Another idea would be to put the sleeve around the piston, so that the seal is at the very end of the soda can sleeve. Put a rubber band around the sleeve at the opposite end to hold it in place then slide the piston & sleeve into the chamber. This way when the sleeve is removed it only travels a 1/2" or so before it is in the chamber. This way there is only that 1/2" area where the seal could be damaged before entering the prepared grease chamber. Just make sure the sleeve is long enough to protrude the rear of the tube when the assembly reaches the chamber area. Like Paul said be sure there are no sharp edges on the soda can sleeve.
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The soda can trick works great for the Trail XL because of the threads at the end of the tube for screwing the end cap on. On an NPSS, there are no sharp threads to get the seal nicked up on. If you do a good job deburring the pin holes and cocking slot and trigger opening, I don't really see a need to use a sleeve to insert the piston and seal. It may be a lot of work for nothing.
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The soda can trick works great for the Trail XL because of the threads at the end of the tube for screwing the end cap on. On an NPSS, there are no sharp threads to get the seal nicked up on. If you do a good job deburring the pin holes and cocking slot and trigger opening, I don't really see a need to use a sleeve to insert the piston and seal. It may be a lot of work for nothing.
Yes, it's a must with the XL's end cap threads. And if it's long enough, it will also help protect the seal if any burred edges along the cocking slot were missed.
Even though I had de-burred all the edges of the cocking slot, I used a long piece of brass shim stock I have to help prevent any possibility of the de-burred edges 'rubbing' a groove in the seal lip, and to make it easier to get the seal past the inner end of the cocking slot on both my Whisper and Trail XL.
It wasn't absolutely necessary, but I had the shim stock, and didn't want to take any chances of harming the seal.
Paul
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OK my parts have finally shipped and should arrive on Tuesday. Now I have a question on what lube to use where.
Do I use the silicone grease on all inside parts of the tube: chamber, and remainder of tube, inside & outside of piston, shaft of nitro piston? Then use the moly on the barrel pivot area.
I've read many post and articles in the GTA Library and it sounded like I should use the silicone grease in the chamber area and gas piston shaft, then moly grease on the inside and outside of the piston and remainder of the tube (from the cocking slot back). Also from what I understand is to just put a fine film of grease on everything.
Any info on this would be great as I would like this seal to last longer than the 1500 shots like the original.
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Great ! Hopefully the Christmas rush won't hold them up also !!!!!!
For a gas ram gun...
I used a very light coating of silicon grease on the tube and seal, based on info I read in a B-19 tune writeup that Charlie wrote. It's worked well so far.
Then, Maccari's moly grease on the ends of the ram, sides of the piston, cocking slot and shoe.
Since then, I've noticed that Maccari on his website, and I believe Gene has also mentioned, also using Maccari's moly on the seal and tube.
Next time I open an AG up, I'll be switching from silicon grease to using the moly on the seal and tube also.
Tough to ignore the advice of folks who've done ALOT of AG tunes for a living. ;D
Paul.
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Well the parts were shipped on 12/17 and they finally arrived today! From NY to NH seems to have taken a long time but with the Holiday season I guess it can be expected. So now that they are here I know exactly what I will be doing this evening. Hope all turns out well!
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Well the Remington NPSS is all back together! A new seal was installed, chamber honed, all possible areas that could damage the seal while installing where touched up and everything got a good lube job. I also hand made a set of brass breech spacer washers and they were installed during this rebuild. I am very, very happy with the results with my 1st attempt to open up my NPSS, not to mention the increase in the Chrony readings as you will see below. All comparison testing was done with Crossman Premier HP 14.3 gr
Original Readings New readings right after tune New readings after 500 shots
701.5 759.9 779.2
721.9 767.5 758.5
717.2 766.4 762.4
712.9 763.9 756.6
718.1 760.3 769.9
721.8 751.8 766.9
721.2 761.0 771.9
717.8 761.6 766.6
718.3 746.4 759.2
708.5 762.2 773.2
Ave. Vel. 721.8 760.1 766.35
Ext. Dev. 20.3 21.1 22.6
Ave. Dev. 4.97 4.44 5.74
Ft-lb 16.28 18.35 18.65
I Chronied several other pellets including the following: H&N HP 12.65 gr., RWS wadcutter 11.96 gr., Crossman Premier Domed 14.3 gr. And the Crossman Domed Select 14.3 and all results showed the same type of increases from before the tune job. Now all I need to do is see which pellets will give me the best grouping.
Thanks to those of you that gave me the useful information that made my 1st attempt at this a success.
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Norm, those numbers look much better and they will probably get better over time.
I find my NPSS like H&N FTT pellets, good velocity and accuracy. You should give them a try too.
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They're on order!
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Now I find out they are out of stock and cannot find any H&N FTT .22's anywhere. Anyone know who may have them in stock?
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I used the old seal and put it on the end of a dowel and put some silicon on it and worked it in and out of the chamber before I assembled the new parts. Every thing went together very smooth and I got very little smoking from the very start of the break in period. It is an easy gun to work on.