Shimming a Diana breech seal

Introduction by B.B. Pelletier

Thanks for all the kind wishes on my eye surgery. It went very well and my exam the next day (yesterday) showed the potential for 20/20 in that eye.

Once again I am running a “Best of BB” because I still could not see the computer screen well enough to write a new report. I saw the doctor yesterday and got the patch off my eye, so hopefully this will be the last time I have to do this.

With all the interest in tuning spring guns I thought you might like to see what effect a new breech seal has on performance. I plan on giving the Beeman R8 a new seal after I get its velocity up higher than it is now. Let’s look at this old report on what a fresh breech seal can do.

I’m also toying with tearing the R8 apart right away and removing most of the new grease, so see if just a little on the mainspring will give what we want. I am really curious about this rifle!

Guest blogger
Here’s another guest blogger. Vince Brandolini shows us how he shims a Diana breech seal to gain more energy. Vince has instructed several readers on this blog how to do this and they report similar velocity gains.

If you have an airgun story to tell, maybe you can tell it here. I’m looking for some budding bloggers who would like to help me write this blog. If you would like to write a post for this blog, please email me at [email protected].

Bloggers must be proficient in the simple html that Blogger software uses, know how to take clear photos and size them for the internet (if their post requires them) and they must use proper English. We will edit each submission, but we won’t work on any submission that contains gross misspellings and/or grammatical errors.

Okay, Vince – take it away!

Shimming the Diana breech seal

By Vince Brandolini

I had gotten my first Diana rifle about 15 months ago – a wood-stocked RWS 34. I found it to be a bit of a disappointment in several respects and ended up selling it after a few months. One reason was the velocity; it generated under 14 ft-lbs at the muzzle. That’s no big deal, but the rifle certainly took more effort to cock than, say, my Gamo Shadow or Crosman Quest, either of which would handily out-power it.

Giving the 34 one more chance
Still, I did sort of miss the rifle. After hearing a lot of good things about the Panther variant of the ’34 (including BB’s review), I decided to take a crack at one. The example I got was a factory-refurbished model, and, frankly, all the reviews were right on the money. It was a much nicer gun to shoot overall, although the velocity was still low.

It didn’t worry me too much until I compiled a list of the mechanical specs of all my springers – including the potential energy stored in the powerplant when the gun is cocked. I then bounced that number against actual muzzle energy and found that all my guns came out at over 30% efficient – except for the Panther which came in at less than 28%. That didn’t sound right to me.

Huge increases in efficiency & velocity!
My suspicions were confirmed when I obtained a .22 caliber Panther a short time later, and it was lobbing Crosman Wadcutters under 600 fps (under 25% efficient). After some investigation, I found that shimming the breech seal resulted in a velocity over 700 fps. That made me wonder about my .177 Panther. So, I shimmed THAT breech seal and the velocity (10-shot average, 7.9 Crosman Premiers) went from 868 to 938 fps – a 17% increase in power and a new efficiency of 32.5%. Incidentally, the efficiency of the .22 is now close to 37%.

Dianas aren’t the only guns that could use a shimmed breech seal
When I found a similar problem with my very recently acquired Ruger Air Hawk (which is a BAM-built semi-clone of the ’34), I began to wonder if this might be a common issue.

How it looks on the inside
Below is what the typical Diana-type breech and seal look like (the pictures are from the near-identical Ruger Air Hawk).

 

The breech reveals a simple o-ring pressed into a groove around the barrel opening. If you remove the o-ring, you may or may not find a factory shim under the ring. Both my Panthers had shims, while the Air Hawk did not.
 

 

When you remove the o-ring, you may find a factory shim under the ring. Both my Panthers had shims, while the Air Hawk did not.
 

 

This is a side shot of the Air Hawk breech. There’s little protrusion from the breech face. That can lead to velocity loss!

Making & installing the shim is just about a no-brainer
This isn’t an expensive or time-consuming process and no mechanical knowledge is required, yet I found it was one of the quickest ways to immediately get a velocity increase. Let’s get to it!

The seal groove on the Ruger Air Hawk has a .540″ outside diameter and a .340″ inside diameter (measurements are approximate).

The tools…

 

I was able to make usable shims with 3/8″ and 1/2″ hole punches. I had a cheap one with interchangeable heads.

The shim can be made out of a variety of materials. Diana uses steel, while Crosman (with a similar setup) uses a thick paper. I settled on plastic from a coffee can lid, which yields a pliable shim about .015″ thick, and aluminum from a soda can, which is about .005″ thick.

Making the shim is a simple 2-step process…

 

First, I used the larger punch to make the outside diameter.
 

 

Second, I used the smaller punch to make the inside diameter.

The same procedure is used regardless of the material. If you use aluminum, the punching process will probably distort the ring, so make sure you hammer it flat before installing it.

 

The tricky part is that second punch. If the tool isn’t centered properly you get an uneven shim that looks something like this. If it isn’t too lopsided, the shim can probably still be used (as is the case with this one), but it’s best to have the second punch centered as much as possible.

What if you need to replace an o-ring?
If you discovered that your gun’s o-ring is torn, gouged, ripped or in some way damaged because that’s how it came in the gun or because you did something when removing from your gun, you can buy a replacement. I checked into Umarex’s replacement breech seals, and found that they charge $4-5 for one. If you need only one, that might be the way you want to go. On the other hand, I was able to buy a bag of 100 McMaster-Carr o-rings for just $3.

 

Clockwise from upper left: yellow plastic shim made from a coffee can lid, aluminum shim made from a soda can, McMaster-Carr o-ring, Umarex breech seal.

Installing the shim

 

Once the shim is punched out, it fits easily into the breech groove…
 

 

…and the seal protrudes far more prominently.

A few minutes of work increases velocity

 

In this case, I used a single coffee-can shim (.015″) to increase velocity from 852 to 871.

Yes, it’s still slower than my Panther, but the Air Hawk isn’t a perfect clone. The powerplant has a shorter stroke (by about 15mm) and a slightly softer spring. Out of the three guns I shimmed, this one improved the least. But, the improvement is unmistakable (it was observed over a series of several shots), and it brought the powerplant efficiency up to a reasonable 32%.

Later, I found a fiber washer from McMaster that seems to work well as a breech seal shim. It’s part #90089A330 and sells for $10 for a bag of 100. It’s about .016″ thick, so it won’t do if you’re trying to shim a very small amount.

This naturally leads to the the question – how do you know if your rifle needs it, and how much should you shim? There’s no set rule. My Diana 350 didn’t need any shims, and frankly I don’t know of any way of telling beyond trial and error. There’s a “wax paper test” for a leaking breech seal: load the gun and put a strip of wax paper between the breech and the compression tube as you close the action. Fire the gun. If the seal leaks, it’s supposed to tear the paper. Perhaps this works in some cases, but it didn’t work for me.

That’s about it. I suspect that my first 34 (the one I sold) suffered from this problem. Plus, Hank wrote that his .22 Panther gun picked up 100 fps.

88 thoughts on “Shimming a Diana breech seal

  1. Hi BB
    Great news about your eye Sir. Thanks to the Good Lord. Your eyes are the greatest assets to you in your profession. So happy that you will be in top form again. God bless you!
    Errol


    • Yepper. Thanks to the Good Lord.

      And thanks to the undergraduate institution with the premed-focused science program.
      And thanks to the medical school.
      And thanks to the medical school faculty, adjuncts and endowment donors.
      And thanks to the preceptor guiding the opthalmologic residency.
      And the institution that hosted it.
      And the doc and team of professionals who busted tail for years to get through the whole science-based system to deliver this science-driven result.

      God Bless the NIH!


  2. That is good news on the eye. A good article and worth looking at any springer breech seal, new or old. This was one I had not seen, though I do remember B.B. making one from an old piece of belt leather.

    Let’s not forget the tissue test, ( laying a piece of bathroom tissue over the breech area ), and looking for it move from any escaping puff of air. The wax paper test is one that I had not heard of before. The tissue test would probably work best for many PCP’s, as more of the breech area is usually exposed. Of course, with PCP’s we are talking an O-ring that seals around the bolt in which a smaller I.D. O-ring would accomplish the same thing.

    Good Day all,… Chris


  3. Been a while back now but I believe there was a trick to finding air leaks around car door seals. Just pat some talcum powder around the suspected area of the metal door where the seal contacts it and very slowly close the door so as not to compress the seal more than it would be in a static sealed condition, If you slammed it closed the powder would transfer but the door would back away from the seal when it rebounds.
    The area on the black door seal that has no talc on it is the part that needs to be shimmed out. May be helpful in this case?


  4. I had not thought of this possible issue. I just may have to take a look at my Diana 46 just for giggles. This would give me a good excuse to play with my new chrony also.

    I am glad to hear that all seems well with your eye surgery!


  5. Going waaaay off subject here.

    I was seeing where you can pre order the new Sig Sauer P320 pistol with the belt fed magazine like on the MPX, etc. and I was curious as to whether the belt is part of the chamber or does a bolt extract the projectile and insert it in the chamber on these CO2 guns? If the belt is part of the chamber, how does the action seal and how well does such seal?

    My whole thought process here is whether this magazine system could be adaptable for use with HPA. With Hatsan, Evanix and others introducing semi-automatic PCP air rifles, a large capacity magazine such as Sig Sauer’s would certainly be a boon.


  6. B.B.,

    How does one adhere the new o-ring to the groove?

    I have vintage BSA springer which I bought sight-unseen online from a prominent air gun dealer who advertised the rifle as “recently rebuilt” despite its missing a breech seal and trigger return spring. I have a vast array of rubber o-rings, but none will stay put with just friction.

    Michael


    • Michael,

      Here are my thoughts that I give to you for free, probably about what they are worth.

      Depending on how “vintage” your BSA is, it may have had a leather seal, which can be easily made. As far as keeping an o-ring in, try to find one that will snugly fit both the inside and outside measurement of the seal seat. That will help to keep it from falling or “rolling” out. You might also want to use a SMALL dab of silicone grease on and/or behind the o-ring to give it a bit of tackiness.







            • B.B.,

              Now I seem to remember learning that once upon a time. :^( Well, if I were unscrupulous I could dump it on someone, but unlike the guy I bought it from, I care about my fellow Man.

              B.B., What do you do with a not-collectable, not functional air rifle with two repairs needed (also needs a trigger return spring), poor metal and wood finish, and likely is not especially accurate anyway?

              Shoot bottle rockets from it every July 4th? Turn it into a low power muzzle-loader? A floor-standing bud vase? A reading lamp? A canoe paddle?

              Michael


              • Michael,

                Why not follow my report and rebuild your rifle as I did? It sounds like you have many of the same problems, and the parts like the breech seal are still obtainable overseas (internet)? The links to the sources are in that report. I was heartbroken to discover what a junker I had purchased, but we made lemonade from it.

                I think I give pretty good instructions to follow for the disassembly. Look here:

                /blog/2014/08/bsa-meteor-part-9/

                B.B.


                • B.B.,

                  Thanks very much for the encouragement. You make an excellent point.

                  I have a vacation coming soon, so I will have some time. It will be a good learning air gun for me to work on. Plus I won’t get nervous that I’ll mess something up!

                  I hope the family emergency turns out O.K.,

                  Michael



    • Dave,

      Here I am handing out my free advice again (see above). My personal experience is these types have breech seals on the compression chamber that goes over the end of the barrel. I have not seen one where shimming would be of much use.

      Once again I will emphasize this is MY personal experience, which is very limited.


      • RidgeRunner
        You could be right! If you suspect seal problems on these kind of guns you would probably be better off replacing the breech seal with a new one.
        Cheers
        Dave


      • Hi RR
        Mind giving me some free advice too? I’m fixing my friends Back packer 2289. Ordered a complete set of seals plus a power hammer spring from GMAC UK. I noticed that the power valve has a tiny hole at the base of the valve stem that gets hit by the hammer. What’s it for? Some kind of safety feature or power limiter? Some of the air would escape from it when the valve opens. Would I get a bit more for if it’s sealed%



          • Errol,

            LOL! I have no clue whatsoever. I have not had the pleasure of playing with very much that Crosman has put out.

            Once again my free advice is worthless.


          • Errol,

            Since you bought the part from Gmac in the UK that is probably a power limiter to keep the pistol’s output below 6fpe. You might be able to fill the hole in with metal epoxy after thoroughly cleaning the part. Just a thought.

            Siraniko



              • Gunfun1,

                Even if you pump it beyond the recommended number of pumps the excess air will bleed off when fired due to the some of the pressure being diverted from the pushing the pellet. I think a similar method is used to decrease the power of springers where a hole is drilled into the piston to limit the amount of air pushing the pellet.

                Siraniko



        • Errol
          That’s the factory valve or is it a aftermarket valve? In the valve itself or the valve stem?

          I never had a 2289 but I had 1322’s and 77’s. I don’t remember seeing that. But it has been several years since I had a 1322 valve out. I have never had problems with them.

          I would like to know more about what your talking about though.


          • Hi Gunfun1
            It seems to be factory. The hole is at the base of the power valve, by the side of the stem. Air would escape from it too when fired. The valve is very similar to the 1322 & 77s.
            Thanks
            Errol


            • Errol
              Maybe it’s a blow off incase the gun was over pumped to prevent valve lock.

              I have a old 1377 parts gun. I need to take the valve out and see if it has it and if so what it does. I don’t know when I will check it out but I’ll let you know what I find when I do.


              • GF1
                Thanks so much. That could be it. A kind of safety against over pumping. I don’t think I’ll mess with it since its not mine. Please don’t trouble yourself by stripping your gun to check. You guys are so helpful, despite all your problems & time constraints. God bless!
                Errol


                • Errol
                  If I remember right I have the end cap off the gun and the striker out of it already.

                  I’m thinking I can shine a bright flashlight in at it and see without taking it out.

                  I want to see if the 1322/77 even have the hole. If it does I do want to take the valve out and take it apart and see if I can tell what it does.

                  And that’s all ok. That’s what I like to do. 🙂




                    • Errol
                      I just took the valve out of that 1377 I have. No hole at all like you described.

                      So not sure why yours has the hole and mine don’t.

                      If you find anything out about it let me know.


                  • Hi GF1
                    Thanks a mil for the info. Looks like my gun has the hole to limit velocity then. Maybe its one made for the Canadian side?? If I decide to buy it off my friend, might try blocking the hole to see what happens.
                    Errol


                    • Errol
                      No problem. And let me know if you do end up owning the gun and plug the hole.

                      But​ did you ever chrony the gun or fire into something soft to see how deep the pellet goes with different pumps? That way you can see if it stops increasing​ velocity after so many pumps.

                      At least you will get a idea if that’s what that hole does maybe.



  7. Hi Vince,
    Thanks for the guest blog. Please explain this; ” It didn’t worry me too much until I compiled a list of the mechanical specs of all my springers – including the potential energy stored in the powerplant when the gun is cocked. I then bounced that number against actual muzzle energy and found that all my guns came out at over 30% efficient – except for the Panther which came in at less than 28%. That didn’t sound right to me.”

    How do you figure out “potential energy max”?????

    -Yogi


    • Yogi,

      Vince dropped off this blog a couple years ago.

      Measure the force of the mainspring and the muzzle energy of the pellet. The delta is the potential.

      Vince’s numbers agree with the Cardews, by the way.

      B.B.


      • B.B.,

        (Not now),…. but you will have to explain how one would find the “force of the main spring”. I would think that a bathroom scale could be used,.. and compress the spring to the cocked height/length. Crude,.. but it would get a reading/data. Like I said, (not now),… but maybe in the future.

        Chris


        • Chris,

          You are very close in your way of thinking, however it could be dangerous to push down on a spring on a bathroom scale as the spring may slip to the side and all kinds of mean, nasty things could happen.

          Drill a hole in your work bench and stand the spring above the hole and run a long bolt through the spring and work bench with a pull scale such as for weighing fish or bow pull underneath. Pull.


  8. Very nice blog post Vince, there is one mistake however, the Ruger Air Hawk isn’t made by BAM but rather Zhejiang Xinhua Sporting Goods Manufacturing, LTD in Deqing (Xisicousa). The factory shims provided by many factories measure .013-5, If you do have to shim a breech seal be careful, too much will give a loose lock up on the Diana type ball detents resulting in terrible accuracy…


  9. Robert Law of the long defunct Air Rifle Headquarters always told his customers a quick way to check for breech seal leakage (barrel cockers) is to load the gun and close the barrel. Then hold the palm of your hand over and close to the breech area, point the gun downrange, and shoot. If you feel a small puff of air then your seal leaks. This may also work for checking the bottom half of the seal, except you cant place your hand that close since you’re underneath the breech with a cocking lever and maybe the forward end of the stock. So I guess when in doubt, go ahead and shim the seal.


    • Reallead
      I have always done the tissue test on air guns. It will detect the smallest air puff. Very easy to see the tissue move. Even if it’s a very, very small movement of the tissue.


  10. No doubt about the importance of the seal. When my IZH 61 gave way, that’s what broke. B-I-L, good point about preserving one eye while doing surgery. I’ve heard that the pilot of Air Force One wears a patch over one eye. That way if a nuclear blast takes out one eye, he can use the other. But this sounds dubious.

    Gunfun1, all those modifications to the cars sound like something out of Mad Max. I could see one of them turning into a big ball of flame. The one thing I liked about the Camaro was that it really looked like Mad Max’s car which I believe was some kind of Ford. I understand the Ford Mustang 2015 has improved visibility which is one thing I like about it. It also has an understated design that is sleeper-like.

    Yogi, why am I not surprised that the Caterham cars are British. When I visited there, it seemed to be their delight to zoom over their twisting little country roads like they were the RAF at low altitude. I’m surprised that there weren’t many more accidents. But that car is definitely not a sleeper. It looks like a dune buggy, and you wouldn’t want to ride one in the rain.

    Matt61


    • Matt61
      The cars that was in the first original Mad Max movie were from Australia. Mostly GM cars if I remember right with some little changes in body detail that set them apart from the American models of the cars. And they were right hand driver’s if I remember right.

      Now you make me want to see the original movie again. Your making me think of some old good times. That movie came out when I was big time into making cars fast. I lived, slept, ate and drank race cars back then. Kinda like I am with air guns now. 🙂

      But definitely had some good times back then.


    • Matt

      I drive a 2014 Camaro , my wife has a 2015 Stang GT . A lot of difference .
      The Camaro has a smaller engine, but it is wired to run . It has two modes of operation ….uncivilized and worse . It can’t do anything slow .
      The Stang is a real pussycat to drive . Very easy. The dealer said that it scares the crap out of him, but I don’t see it . Maybe the 440 horse 5.0 .
      The Camaro is just plain nuts .

      tt


      • TT
        I was thinking about you the other day when I replied to Matt about the Camaro’s.

        How can I say this. I guess I’ll just say it. I don’t like the design of the newer Camaro’s. But they do run. Even the V-6 like you got in yours. My nephew has the SS with the LS1 based 396. And yes it runs.

        So to say. Like I told Matt to the other day. If somebody wants to give me a new Camaro I would for sure take it. 🙂


        • GF

          They be about the worst rolling blind spot I have ever seen . And twitchy .

          Have to wonder if the SS and Z28s are as bad with the auto transmissions . The computer in mine is crazy.

          tt


          • TT
            Even the manuals have the dreaded skip shift that blocks out certian gear shifts.

            That’s the thing about all this computer stuff on the new cars. It could be good for certian conditions but haunt you in others.

            I luv’em and hate’em at the same time.


            • GF

              About the only way I can get my car to almost act normal is to go into sport mode, then activate the paddle shifts on the wheel and shift manually . A bit of a pain with a 6 speed .
              It still shifts down if it thinks the rpm is too low .

              tt


              • TT
                All I can say is the steering wheel paddle shifters are way cool.

                That should be available as part of the standard equipment on every car. No if and or buts.



              • TT
                Yep the skip shift was used on earlier model Corvette’s before the Camaro’s.

                Alot of driver complaints that they couldn’t drive the car how they wanted.



  11. I’ve been building myself up to a guest blog either about the Anshutz 335 or the use of shooting in my sons therapy
    Amusingly it’ll be my use of “proper English” that may cause the most issue 🙂
    I think I’ll have to put spellcheck on and set it to US English 🙂


  12. You know you’re a redneck when…………….

    1. You let your 14-year-old daughter smoke at the dinner table in front of her kids.

    2. The Blue Book value of your truck goes up and down depending on how much gas is in it.

    3. You’ve been married three times and still have the same in-laws.

    4. You think a woman who is “out of your league” bowls on a different night.

    5. You wonder how service stations keep their rest-rooms so clean.

    6. Someone in your family died right after saying, ‘Hey, guys, watch this.’

    7. You think Dom Perignon is a Mafia leader.

    8. Your wife’s hairdo was once ruined by a ceiling fan.

    9. Your junior prom offered day care.

    10. You think the last words of the Star-Spangled Banner are ‘Gentlemen, start your engines.’

    11. You lit a match in the bathroom and your house exploded right off its wheels.

    12. The Halloween pumpkin on your porch has more teeth than your spouse.

    13. You have to go outside to get something from the fridge.

    14. One of your kids was born on a pool table.

    15. You need one more hole punched in your card to get a freebie at the House of Tattoos.

    16. You can’t get married to your sweetheart because there’s a law against it.

    17. You think loading the dishwasher means getting your wife drunk.




  13. I had to laugh at the complaint that the RWS Model 34 was not as fast as a Ruger. Air rifles are designed to achieve certain design parameters, and complaining that one pushes a pellet faster than another is like complaining that a VW microbus will not go as fast as a Shelby GT.

    I have a quite old Diana Model 35 and when I bought it there was only a slight remnant of the original leather seal still in place. Once I replaced that, and found a decent rear sight, it is one of the most accurate springers I own. Not a barn burner, but it shoots better than I can.


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