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DIY How do you know…?

How do you know…?

by Tom Gaylord
Writing as B.B. Pelletier

This report covers:

  • August’s question
  • Experience
  • How do you tell…
  • Expected power
  • Piston stroke
  • Age of the gun
  • Chronograph or other means of power determination
  • How do you know? — case 1
  • Case 2
  • Last point
  • The last word

Today’s topic tries to address a question I am sure many newer airgunners have at some point. How do you know when a spring gun need repair? It was asked last week by reader August, who lives in Germany. Here is what he asked.

August’s question

”How do I recognize that an older gun piston seal is going bad? From reading the blog I gather that I can chrony it. But this gun delivered until right before the final breakdown. Only the last five shots it became slower. On opening the gun I saw that the outer part of the plastic seal was detached from the rest and had blocked the spring tube probably causing the older spring to break.

I am not looking for the scientific way of knowing, but more the gut feeling about it. What is the moment when you handle a gun and you think: I have to look at that one, it is nor feeling right.

That’s a pretty good question, don’t you think? I could answer it with a single word — experience. But that’s not what August wants. He wants to know how someone with experience knows whether a piston seal in a spring gun is good or if it is about the fail. That’s what we will look at today.


Let me get this out of the way first. Experience does matter in these cases for several reasons. First, I know there are certain spring guns whose piston seals were made of a material that deteriorates with time. It doesn’t matter how much they have been shot, the original seals will always go bad after a certain amount of time has passed. The FWB 124 (and I assume the much rarer 121, as well) has this problem. So do the early Walther spring guns. You’re going to encounter this problem in the Walther model 55 most often because more of them were made, but the LGV had the same problem.

Moving on, all the Diana recoilless airguns — both rifles and pistols — had seals that go bad over time. That would be the models 6 and 10 pistol, the breakbarrel target rifles (models 60, 65 and 66), as well as the various versions of the model 75 sidelever target rifle.

The good news is that all of these airguns have had their seals reproduced in modern permanent material that lasts indefinitely, so they can all be repaired. Some, like the Walther LGV, may be harder to find, but I have bought them over the years, so I know they are out there.

Hakim trainers, made by Anschütz in 1954 don’t have the same problem because their piston seals were made of good material. But many of these rifles were abused by the Egyptian military and you will find their seals destroyed by smashed pellets, sand and nails embedded in them for decades. Fortunately a modern Diana piston seal can be modified to work in the Hakim so, even though the original seals are no longer available, they can be repaired.

How do you tell…

Okay, experience helps. But what if you are looking at an airgun that you know very little about? What are some things you look for?

Expected power

I could probably write a chapter about this one — the power output I expect an air rifle to have. There are things that I always consider.

Piston stroke

How far does the piston move when the gun fires? You can often tell this from how far the barrel breaks down, or how far the sidelever or underlever moves when the gun is cocked. For simplicity I will address a breakbarrel.

If the barrel breaks down halfway or less (90 degrees), the gun has a short-stroke piston. The power will be on the low side. The Diana 35 was an excellent example of this. It has a large diameter piston and a powerful mainspring, but the stroke is so short it can never develop much power. When the Diana 34 came out (a replacement for the 35?) it had a much longer stroke and way more power.

A Diana 35 in .22 caliber was supposed to generate 11 foot pounds. You can read about it in this report — Diana 35 — Always the contender. If this software permitted subtitles the one for this report would have been — Never the champion.

Another rifle that never met the power potential people thought it should was the HW 35. Again, the stroke is too short to generate much power. The HW80/Beeman R1 that has the same piston in a longer stroke action was the first mega-magnum. That’s what the stroke length can do.

Age of the gun

If the airgun in question was made in the 1960s or earlier, it won’t have much power. That is a given. There are several reasons for this — leather seals, small transfer ports, off-center transfer ports, short strokes and pistons with smaller bores. A BSA underlever made in 1910 is usually about as powerful as one made in 1960. The technology just didn’t advance very much during those years.

I could go on but you get the idea. When an airgun was made often determines the power we can expect from it. But the power potential wasn’t August’s question. He wanted to know how hje could tell when a piston seal had failed or was about to fail.

Chronograph or other means of power determination

You have to have some means of determining the power of a gun. When it gets bad and the pellet won’t even leave the barrel, it’s pretty obvious that something isa wrong. But how can you tell the difference between a rifle that shoots 540 f.p.s. and the same one that shoots 760 f.p.s.? They may both feel the same. In fact, you got to see this when we looked at the Beeman R8 recently. If I didn’t chronograph that rifle I would never have known what it was doing.

Sometimes it’s not the piston seal that’s at fault. Diana breakbarrels are notorious for breaking their mainsprings. Typically they break off about an inch of the coils on one end or on both ends. Those coils then wind themselves into the remainder of the spring and the rifle continues to shoot smoothly. The only way to detect this is either with a chronograph or by measuring the cocking effort. It will decrease by several pounds when the spring breaks. The piston seal in these rifles is nearly indestructible, but the mainsprings are sometimes hardened too much and they can break.

How do you know? — case 1

Okay, let’s say you have a chronograph and you just bought a vintage airgun. It’s a Diana model 27 in .22 caliber. You chronograph it and see that it shoots RWS Hobby pellets at 375 f.p.s. Your research tells you this rifle should shoot Hobbys around 500 f.p.s., or so. Now you know something is wrong. What could it be?

The Diana 27 has a leather piston seal, so the simplest thing to do is to oil the piston seal and see what happens. You do and the velocity increases to 412 f.p.s. for about 50 shots, then it drops back to 375 again. Something is wrong.

My guess is the mainspring is worn out. Springs have a life; leather seals can last for decades. You won’t know until you disassemble the rifle, but I would bet on the mainspring.

Case 2

You are interested in a Theoben Eliminator in .25 caliber. The chronograph says this one shoots a 25-grain pellet out the muzzle at 635 f.p.s. That works out to 22.39 foot-pounds from a gun you expected to produce 31+ foot pounds. Something is wrong.

This rifle cocks with 68 pounds of effort, as measured on a bathroom scale. That tells you what the problem is in an instant!

Sixty-eight pounds is too much cocking effort for a Theoben air rifle. The gas spring has been over-pressurized in an attempt to get more power from the gun. By doing that they turned the piston into a slide-hammer that slams forward so fast it overheats the piston seal. Over time, the seal is vaporized, leaving a hole in its top. That hole decreases the amount of pressure the piston can generate, and that decreases the power.

burned piston seal
This piston seal was vaporized by over-pressurizing the gas spring of a Theoben rifle. There is a deep hole in the seal where it was vaporized over time.

Last point

August, I just made the biggest point of this report. If the piston seal is leather, chances are it will still be good. I have seen leather piston seals that were over a century old and still functioning. If the seal is synthetic, though, there is a better chance it is at fault when the power drops. And this relates to the airgun’s power.

Higher-powered airguns are more likely to have piston seal problems than lower-powered ones. And leather seals last longer than synthetics, though they often don’t develop as much power.

The last word

So — how do I know? I don’t. It’s that simple. If I have a gut feeling, it will be based on experience, and I have given you examples of that in this report. There are very few warning signs for a piston seal that’s about to fail.

Using the experience I have tried to explain in this report, I can sometimes make an educated guess, but until the gun comes apart I don’t know for sure. Show me an FWB 124 that’s shooting pellets very slowly or not at all and, based on my experience, I would guess it’s the piston seal.

46 thoughts on “How do you know…?”

  1. Nice. Very nice. Just like a good mechanic. Evaluate the symptoms based on experience and then proceed from there.

    For those without a chrony,… saving some old targets may be of some benefit. If the rifle starts shooting 2″ low @ 25 yards, after shooting on target for the last 10 years,… well then,.. something might be going on.

    Good Day all,… Chris

  2. The lower power level of the short stroke of the HW35 is one of the reasons I have hesitated to buy one of these classics. I had been wanting a sproinger with more power, like an HW95, but I really like the features they offer with the HW35. Now that I have the power range covered with my Tomahawk, I should probably give some more thought to one of these.

    • Greetings RidgeRunner
      I understand your hesitation concerning the Weihrauch HW35. After all, the original model debuted around 1952-3, and aside from a larger piston diameter, the gun has essentially remained the same model you can purchase brand new today. However, it has a few unique features that still make it a top seller, and a very desirable rifle. I have the HW-35E model in .177, that comes with a walnut stock, sling swivels, and the unique barrel lock device that is smooth, and almost silent to operate. Mine likes H+N FTT pellets that give me just over 700fps. Not a screamer by today’s standards, but it will give you half inch groups at 25 meters. Another plus is the lack of pre-load on the main spring, thus negating the need for a spring compressor when disassembling, and assembling the gun. I rank it in the top three airguns I shoot most often. My new HW-30 in .22cal currently ranks number one, and the new ambidextrous stocked HW-77 is number two in either .177, or .22cal. I believe it was also the first model to sport the Rekord trigger, which speaks for itself.
      Although I feel that recommending something as personal as an airgun is not quite the equivalent to recommending someone invest in a certain stock during a bull market, I do feel confident the HW35 will meet, or exceed your high standards concerning “sproingers”. Lastly, being a Weihrauch it should provide you a lifetime of service, as well as being a nice hand me down for the grandchildren if, or when you should choose to do so.

      • Titus,

        All of the reasons you listed are the very reasons I have wanted one. The one thing that has held me back is the supposed anemic power level. I have been wanting something with a little bit more horsepower than the HW35 puts out. I now possess a Webley Tomahawk / Hatsan 95 that has the horsepower I have wanted in a sproinger. Now I am giving consideration to a quality sproinger and the HW35 is at the top of a very short list. In fact, the only real competitor it has is the Diana 340 N-TEC Luxus. The HW35 has a slight edge, but not by much. I guess I will just have to get one this coming year and the other the next. 😉

  3. Interesting seal design in the photo above. It appears to have a very deliberate impeller design influence, same as what would be found in a water pump. Ideas?

    • Chris,

      It looks like Theoben was trying to create a vortex of swirling compressed air at the end of the piston stroke. I guess they thought that would help the air pass through the transfer port faster (or slower?). But I’m just guessing.


      • B.B.,
        Creating a vortex makes sense as the compressed air would pass through the transfer port much faster..
        Fill a 2 litre pop bottle with water then turn it up side down and rotate it quickly in a clockwise direction to create a vortex and the water will pour out the narrow neck much quicker than if you just hold it up side down. Also, in Karate, you generate more power in your punch if you rotate your first and arm as you strike- more power for the same amount of effort.
        I would think that the piston and the seal would have to rotate to achieve the desired effect.


        • Pete,

          I think you might get a vortex of sorts just by pushing this shape forward. That,s how snow reacts to a forward moving plow blade. It curls up and forward back over itself as the cant of the blade directs it toward the edge of the road. Cant it in the opposite direction and it would move towards the center of the road or center of the compression chamber in this case. That might create a vortex, don’t you think ?

      • B.B.,

        See reply to August below. I liked the innovation and thought behind it. It looks like something I would do/try if I was an air gun designer. Of course, my inclination would be bearings, multi-stage impellers, boosters, etc.. Second thought,.. I better just stick to air guns as a hobby. 😉

        I do have German descent, so maybe there is something to the saying that they are known to have over engineered things?


        • B.B., Pete, Halfstep, and Chris,

          I have no comment about the Theoben system, but I once firsthand witnessed the Coriolis Effect in a Quito, Ecuador wash basin drain. :^) Lefty Lucy.


      • Chris USA & B.B.,

        I would guess the spoke pattern in the face of the seal is to add rigidity to the relatively soft seal material. Keeping in mind the power level of the Theoben Eliminator, when the gas ram is pressurized to the max the piston (which is probably on the heavy side also) is going to be slamming into the end of the compression tube very hard, even under ideal conditions. I bet a typical seal of the day would not hold up, it would simply be smashed to death over time. That rifle was and still is a beast of a spring gun.

        David H.

        • David,

          I don’t know how true it was, but Davis Schwesinger who gave me that seal told me he has seen seals from Eliminators with 50,000 shots on them, and, except for discoloration, they appear new. But Theobens are often over-pressurized by their owners, just because they can.


          • B.B.,

            If that is true then whoever designed the seal got it right! What I have a hard time believing is that someone actually cocked that big hulking thing 50,000 times! Must have been a young kid, with energy to burn.

            David H.

  4. BB,

    Thanks, this was quite illuminating. Basically it is: Know your airgun (as with my Walther 55 where the seals are a problem) and build up experience. And there is no shortcut to that.

    If I should have directly stopped after the first two slow shots, I could have saved my mainspring. And I better get the tools out and check the other 55 and LGV as they are prone to the same problem. And if I buy something I have to read about it, then I should have known what to look for.

    Ridgerunner, I like the HW 35 and similar types of airguns (the older HW 80, the Diana 27) better in .22 version than in .177. For accuracy they are the same but they handle better. Less nervous I would say. Has anyone the same experience or a difference opinion here on the blog?

    CrisInUSA, It looks more like a disadvantage than an advantage. The seal is burned right on the place of the fin. That is to be expected as a small ridge of material will heat faster that a bulk. Maybe they designed the seal like that in order to save weight or material, but that would account to much in my opinion.
    Do you know whether these seals can rotate in a Theoben? If they can then the wear would be more evenly distributed along the rim of the seal.



    • August
      I’ve had the same experience with preferring some of the older guns in .22 caliber. Way back when my only air guns were a FWB 127, FWB 124, and some other brand of German rifle (probably a Wischo that I believe was appropriated by a relative long ago), and a Webley Premier Mk II, my favorite rifle on several rabbit hunts in Colorado and one on San Juan Island, here in Washington, my favorite was the FWB 127. I cannot really give you a good reason why, it just “felt” right.
      However, I did not find the same with the Webley. I purchased a .22 barrel for it and changed barrels frequently but would have given a slight edge to the .177 barrel. I’m currently experimenting with a Beeman RS2 gas ram and gut feeling leans me toward the .22 barrel.
      Many, many years ago I seem to remember an early air gun writer that made the statement that when comparing the two calibers with exactly the same power plant, the .22 would be more “efficient”. I never fully knew what was meant by that.
      Larry in Algona

      • LarryMo,

        I have said that several times. Efficient means better at doing what it is designed to do — develop power. In a given powerplant a .22 will be around 20 percent more powerful. I can prove that with a Whiscombe spring rifle.


        • B.B.,

          My understanding of that is ,..foot pounds at target. Yea,.. it may be bigger,.. it may move slower,.. BUT,.. it will have more “punch” at target. I think you mentioned a 10# cannon ball moving at 300 fps to illustrate that point at one time.


    • August,

      I do not know if they rotate. Theoretically,.. the spring rotates,.. so if the seal is affixed solid, it would rotate. Either way, I thought that it was an innovative idea. At the minimum, I would think that the transfer port/end of compression cylinder would have be fashioned in a funnel like manner to take full advantage of any vortex created.

  5. Great article super informative ! I’m surprised no one has mentioned keeping a log for each gun, especially when one has guns that don’t get shot as frequently as others. I was thinking what a person could use in lieu of a chronograph to establish a base line, and the only thing I could come up with is some sort of penetration test like a bar of soap, an old phone book, or a deck of cards.

  6. BB

    I just started testing pellets for accuracy from my new .22 Marauder, beginning at 10 meters, using the out of box factory tune, which seems to be around 2900 psi -2000 psi. I have not found any wadcutters that are remotely accurate and I’m assuming that their 940+ fps velocity is at least partly to blame. Is there a velocity that you would recommend for wadcutter testing ? If I were to just shoot the gun down to a lower and lower pressure as I tracked the group size and velocity am I likely to see a better velocity range for these pellets with the gun tuned as it is or will the pressure fall off too steeply as it goes under 2000 psi to tell me anything. I’m using a hand pump and really don’t want to take the pressure way down if this is not a viable method because pumping seems to be causing some issues with the tendons in one of my palms. I also am not nearly educated enough yet to start tuning the gun down to a lower velocity / power level but plan to eventually, especially if I found that the gun would shoot wadcutters like a laser, for instance. Thanks for any insight you can offer.

    • Halfstep,

      Why shoot wadcutters at all in a Marauder? They don’t give any advantage that I can see.

      There are Marauder owners who have published tests with their rifle’s pressurized down to 1000 f.p.s. (like 1800 to 1000) and they say they get stable results. The rifle is so flexible that you can do anything with it, but if it is set up for 2900 psi, then shooting below 2000 isn’t going to be good.

      Two things govern the lower power of the gun — the strength of the hammer spring and the size of the air transfer port. These can be adjusted separately and will each have demonstrable results.

      Here is a discussion of tuning the striker spring tension:



      • BB

        I guess I just thought that if they shot well it could ultimately be a cheap pellet variety and also in certain situations (friend or family member’s back yard, perhaps) I might just utilize the poor ballistics of a wadcutter to add another layer of safety since they should ( I believe) fall away in a shorter distance than a domed or pointed. I didn’t buy the Marauder to hunt with or control pests (although in some cases a wadcutter might also be best for that purpose as well), just for accurate plinking.

        Thanks for the link. I read the stuff on the .177 and .22 when I was trying to pick a caliber for my new gun but I skipped the .25 at that time. Then I guess I forgot to go back. ( I’m also trying to catch up on 11 or 12 years of your blog. Can anyone say “prolific writer” ? )

      • Halfstep,

        If you don’t have a chronograph yet, you may be able to find the info you need about reducing the velocity through the hammer spring and transfer port adjustments by talking to the techs at PA. They may know the exact number of turns on the adjustments to get it shooting to a certain lower velocity OR to maximize the valve to work efficiently at a lower pressure range.

        David H.

        • David H.

          I have a chronograph and will be using it at some point to make power adjustments on the gun but I want to educate myself better first. I thought shooting the gun down to a lower pressure as I watched my group size might tell me what power level to aim for but BB says this probably won’t be effective. If I understand him it’s because the velocity will be so far below the tuned velocity that the valve and striker won’t work well together to produce consistent shots.

          • Halfstep,

            Just to complete the thought I gave, I do believe that it is possible to tune the Marauder to shoot at lower pressure levels. I said I didn’t think the way it is set up at present would work with shots below 2000 psi.


            • BB

              You made that clear before and I understood you perfectly. I figured it could be tuned down since DIY tuning is sort of it’s claim to fame. I’m just not ready to undertake it yet because I’m still exploring the factory tune and I just want to study on the process a little more before I try it.

  7. I have a Hy-Score 807 in .22 that I have tuned a couple of times. I can’t get the velocity above about 430 fps, even with a Merlin XL spring. The barrel has the Freimark marking (F in a pentagram). Did Hy-Score actually market Freimark airguns and will it have a tapered compression chamber like my FWB121?

    • raveller9,

      Welcome to the blog!

      The 807 was a Diana 27 and that was always Freimarked. The Germans just changed the name on the gun but the Freimark remained.

      Never heard of the compression chamber being tapered, though. Don’t think it had to be. The stroke was short and the piston not that large.


  8. B.B.,

    Three quick lubrication questions:

    1st, Do synthetic springer piston seals require lubrication and if so, what kind of oil?

    2nd, With a vintage, leather-sealed medium-powered (say, 750-850 fps.) springer, what sort of oil should one use? (I would guess a high flash point silicone, but better to ask an expert.)

    3rd, With a multi-pump that refuses to compress air (little resistance, mild wheezing sound during the pumping), what type oil should one use to try to revive it before sending it off for repair?

    Thanks as always,


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