Breach seal shimming questions

[bradyman1]

First how proud should the breach seal stand from the surface? Secondly what should be used behind the breach seal for a shim?

Thanks

[amb5500c]

1. Just enough to do its job.
2. Dental floss works if needed.

Richard

[bradyman1]

So as long as it is above the surface it's good?

[GBertolet]

You need just enough of the surface exposed, to seal off the breech, without interfering with barrel lockup. Sort of trial and error.

[ezman604]

I use Crosman breech shims. They are about $1.15 each. Part # BT1500-01-100 for XL1500 and BT1122-01-100 for XL1100.
Happy Shooting!!!!
Dave
 

[john]

My TrailNP22 is ~4mo and ~2K rounds old…no mods o/t GRT3. The breech seal protrusion has become sloped from thick down low to thin up top. My idea is that that is the shape it likes, it seals good, if it stops sealing good my new chrony should pick that up. The seal has taken(adapted to) the shape of the gap between breech and breechblock…that is exactly what it is designed to do and why an elastic material was chosen. Frankly I'm amazed at how well the seal performs what I assume is it's intended function. Why fight that with a shim…isn't that treating a symptom of a problem without addressing the problem (assuming there is a problem which I don't really see one). If I shim that slope out, the seal is just going to get smushed even more over time. I say don't fix what ain't broke. And I've said more than you wanted to hear. Hit me back how I'm wrong or missing something…?...

Once Crosman sends me a backup (hope they havn't lost the order), I will take mine out just out of curiousity. Then put it right back in unless, at that time, I decide to do the droop fix in which case I would install the new one because the fix, if it works, would change the gap in which case I would want to see how a fresh seal adapts over time.

[digger]

The seal groove on my Ruger Airhawk is so deep the o ring is flush with the breech lol. Being new to air rifles I didnt notice it but when I started seeing a shiny deposit on the breech face (seemingly from blow by) I figured out the problem. I used a single wrap around of brick masons nylon line. That got it up about 1/3 out of the groove. So far so good.

[Bryan Heimann]

sometimes vertical stringing in your groups is diagnosed as a bad breech seal.  sometimes a little air escapes, sometimes it doesn't, so velocity varies enough to string your groups vertically.  might need a shim, might need to be replaced.  then again it might be due scope shift, or the piston seal, or even scratches in the cylinder wall itself.  or maybe even bad breath control.  you'd be surprized how many people out there don't know about the importance of breath control for good marksmanship.  but aaaanyway, shoot a few groups at like 30 meters, that should long enough distance to notice vertical stringing.  if your groups aren't spread out mostly vertically, then your breech seal is probably working fine.

[wicki67]

ok, not to hijack the thread; but, what is the best way to get this bad boy out of the breech? Obviously, without tearing it up....

Dave T.

[john]

Dave, Here's what I did for the first time yesterday. Soaked the seal in isopropyl alcohol (better known as rubbing alcohol available at any drugstore). Makes it slippery. Harmless. Then used opposing dental pics down between the outside of the seal and the outside of the breechseal channel it sits in. Gently worked away at it trying not to damage the seal as it is my only one. Was able to get it out with minimal cosmetic damage. Lemme know what you find. Mine looks perfect. It is not smashed or anything. I'm going to put it right back in. Waiting on a spare from Crosman. If you get a chance to measure the depth all the way around of your breechseal channel/grove/whatever, please do because MY channel is a hundredth shallower below than above. Just wonder if that is common. Thanks. John

[wicki67]

John,
Thanks for the insight on the seal removal.... Actually, was looking at how this was done, as I have not touched on breech seal replacement on any of my guns yet. Thinking of it as preventative maintenance, should the need ever arise.... Thanks again for the help!

Dave T.

[EJB]

Thanks for all the good info.  I am going to order shims to keep on hand for all my air guns.

[uncle paulie]

You can use a jewelers screwdriver to remove the seals. The ideal depth the seal should have is whatever is necessary for the gap between the breach and receiver to be parallel. This will allow perfect alignment between the two and make zeroing in your sights easier. If the gap is smaller at the top, the whole seal is in too far, if at the bottom, out too far. The POI is affected by any change you make, and will need adjustment after any changes.

pv

[dkwflight]

Hi

I am not sure I agree with the last post.
If the breach seal is holding the breach away from the cylinder the position of the barrel will vary up and down won't it?
Wouldn't metal to metal contact be more uniform?

[john]

He apparently believes that the seal is strong enough (dense enough) to change the gap between the breechface and the action face (transfer port face...what is the proper term?). I just happen to be investigating this today. It gets complicated the further I look into it.

When my barrel is locked up, there is no gap on top. The fact that the bluing is gone from the top of my breechface is consistent with that. It's bare metal there where the top of the breechface hits the action (cylinder face, whatever) when locked up. A feeler gauge stuck up from the bottom indicates about 8thou gap on the bottom. Therefore, the gap between faces is not parallel at lockup.

And yet, when the barrel is broken, the seal protrudes above the breechface. That is one heck of an elastic memory considering the seal spends the vast majority of its time smushed. No evidence of leakage from mine. I've done all the hokey tests. Kleenex, baby powder, you name it.

Could I shim the seal enough to bring the faces parallel? Yes. That would definitely increase droop...undesirable. That would also increase the vertical misalignment between my breech port and transfer port...undesirable...i have posted elsewhere* that they are not coaxial.
*http://www.gatewaytoairguns.org/GTA/index.php/topic,13756.0.html

I could go on...and I will. I measured the depth of the breech seal channel. Guess what? It slopes...it is one thou deeper on top than bottom. That is why my seal is one thou prouder on bottom than on top when the barrel is broken...all easily measured with the depth probe of a Vernier caliper.

Several questions arise.
Is mine a one-off or all they all that way?
Did Crosman do that on purpose?
If they were going to do that on purpose, how the heck do you machine a circular channel whose bottom is sloped? Think about it? computer-guided laser? Drill at an angle? Would they spend the money for EDM? Highly doubt it. They maybe drifted in a pie-shaped shim.

Whadya thimk?

edit1: p.s. i forgot to mention that my gun is shooting fine. Based on above i see no good reason at this time to change the gap, especially since the side effects IMO are clearly undesirable.

edit2: http://www.gatewaytoairguns.org/GTA/index.php?topic=17738.msg153909#msg153909

[uncle paulie]

Not sure why they would manufacture these things to have an uneven gap between the breach and the receiver. Seems to me the correct position of the barrel and the receiver should be a straight line. If the gap is smaller at the top, you are creating the reverse of barrel droop, which logic would indicate is also undesirable. I haven't had the opportunity (need) to replace my breach seals, so have not put this to a test. Perhaps one of the Guru's will chime in on this one.

pv

[3ants]

2 of my crosmans breach seals was sticking out the bottom more than the top as i looked at it thay cut the breach at an angle the top would rub the blueing of the piston chamber i took a big flat file and took of the angle now when it locks up the the breach rests on the alive jam cross pin instead of rubbing the compresion tube and the seal looks even it rasied my poi alittle had to resight alittle seams to be working ok

[S.S.Tupperware]

Its actually quite simple... The detent to hold the breech closed needs a solid stop on top, hence the blueing wearing off. Otherwise it would be supported by the O ring, and well with a soft material and recoil... well.

[john]

In an elastic lockup scenario, i would not be surprised to see vertical stringing.

[3ants]

if your blueing is rubing off your piston chamber look at it after 500-1000 shots i could see metal starting to disapree above where the seal strikes the piston chamber now the breach strikes the cross pin and the alive jam locks to the cross pin metal to metal lockup the seal now seals in stead of being the stop