breach seal shim height???

[cfxdragon]

I got my brand new RWS 350 last night and when I took it out of the box and gave it the once over I noticed that the breach seal looked almost flush. Today I got out the straight edge and feeler guages and found that it is from flush to just a couple of thousandths above the surface of the breach block. I plan to shim the seal before I even fire it the first time, no sense in damaging the piston on a brand new gun. I am trying to find out what the proper o-ring height should be so that I can place the correct thickness shim under the o-ring. I have searched and searched and cannot find how much the o-ring should protrude from the breach block when properly shimmed.

Does anyone know a spec, or can take a measurement from an RWS that is performing properly?

Thanks in advance guys.

[davee1]

A long time ago, I asked this very same question. No one could answer with a specification. I understand why now. Because its different for each breech. I ended up measuring the velocity with a chrony, without shimming...then shimming and measuring the shim's effect on velocity/consistency. There is a point where a good breech seal is made, velocity cannot increase anymore, and more shimming will affect positive lockup. In my experience, a few thousandths to 10~30 thousandths may be needed. Experimentation is key and a chrony is a must. More than one shim can be used, and shim material can be made from many sources. I like gardners tape...that green vinyl tape that folks use to tie trees/branches up to a stake.

[cfxdragon]

Thanks for the reply davee. I ended up modifying a nylon washer for a shim. It looked pretty good and lock up was fine. I thought the o-ring should protrude more so I tried adding the factory shim back in also. That was too much however and just slightly cut the o-ring, so I took it back out and replaced the o-ring.

I understand that there is no set shim height as that is the purpose of the shim is to take care of machining differences. I would have thought that there would be a set height for the o-ring above the surface of the breech.

I don't have a chrony and there is no way I am getting approval from the finance department for one after the purchase of the 350 just this last week... Maybe later next spring :)

[onemountain]

The breech seal on my B28 protrudes so little I have to look at it in profile to tell that it's actually raised above the surface of the breech block. But it chrony'ed fine over at Gene's about a month ago when he tuned it. So I guess it's one of those things that changes gun to gun in airguns, just like every other miserable detail lol. That said mine does make me a bit nervous, so I think I'll shim it just a wee bit to be safe.

[tjk]

I've used aluminum from a soda/beer can. Cuts easy with sizzors,...and a set of hole punches work just fine. Not sure,...but I think the thickness of most cola cans is about 1/1000". Good luck. tjk

[davee1]

Todd,

No chrony? Oh no!!!! Same thing happened to me. Squawking from the finance department about the initial outlay...and now more stuff? More about that later...

Nope...there is no spec that I know of for seal height above the breech surface...I think because of all the possible tolerances from all of the components that cause a "breech gap". So you have to experiment. Experimenting without a chrony can be done with a medium such as an old telephone book or a nice block of electrical duct seal. Measure the depth of the pellet in the paper or duct seal with no breech seal...then add a shim and measure depth again to see if the pellet went in a little deeper this time. Chances are, a slight shimming will increase the velocity, thereby increasing the depth of the pellet channel in the target. Another thing to play with is actually replacing the breech oring with another slighty larger one. I have good luck with #8 orings(plumbing size) like what you find in the faucet parts isle at Home Depot. I also ordered a bunch of orings fromMcMaster-Carr, size 109 and 110 made from Viton material. I ordered both of those sizes because I thought if one didnt fit then surely the other would.Then I ordered a whole bunch of different sized orings that would seem to fit in the breech seal cavity of all of my airguns, so I could find the exact size thats right for each one.

When seeking the finance department's approval, its always helpful to point out the sheer amount of purses, shoes, dresses, skirts, blouses, makeup, and god knows what else. Each item has(had) its purpose...except with airgunning, a chrony has many purposes and will be used over and over...not stuck in a box somewhere never to be used again.

[cfxdragon]

That's the problem though, the head of the finance department is very generous and not into those purses, and shoes.

On black friday she even bought me the last rifle cabinet I needed to complete my collection and also a new soft sided bow case. I had mentioned when she was looking through the adds, after turkey day dinner, and talking about going shopping that I was going shopping in the woods (deer hunting) and that the only two deals I saw were the cabinet and the bow case. When I called her that evening to tell her I was bringing home groceries (about a 200# FD doe) I found out that she couldn't get most of the things she went to get but picked up both of my black friday deals for me.

So I don't think I should push anything for a while. I'll have to wait or squirrel away a little money here and there.

[cfxdragon]

The one on the left is the new one. The one on the right holds my bow and the Genesis bow for the boys.

Now I just need three of the pistol cabinets to mount on top 8)

[davee1]

Oh my...your head of the finance department is extremely advanced. Yeah...dont push it is right. Put it in next fiscal years budget I guess.