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.