Piston seals: Part 2
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Importance of round air chambers
- My best advice
- Out of round pistons
- The deal
- Plain seals
- Leather piston seal assemblies
- Parachute seals
- The PTFE seal
- Last subject — seal materials
- Piston rings
Today we will continue our discussion of the piston seal. This report was prompted by the comments of new reader Arvizu, who said, “ I noticed, too, that the seal plays an important role to define performance (sometimes small variations in diameter makes the difference). I would like to clarify that this is only my appreciation and limited experience with airguns.”
In Part One I attempted to show how a piston does its job. Arvizu’s question seemed to imply that he thought there might be an unlimited horizon for the piston seal, if only the technology could extract it. I tried to show that a piston is limited by the volume of air it compresses and improving piston seals is asymptotic. In other words there is a limit on the power that’s possible and we are now shaving small percentages as we approach that limit. To put it bluntly, we will never see the power of a spring piston system doubled by the seal. Never!
That being said, I’m not ready to say we have reached the end of improvement. But this report is not going to address that. We are looking at what the piston seal does and at the different types of seals that have been developed.
Importance of round air chambers
One comment that came in last time from reader Kevin reminded us of the importance of a round compression chamber. If you don’t have that, there is not much that can be done to improve it. I have owned spring rifles with out-of-round chambers and they definitely did stall out at a certain power point. It only takes a few thousandths out of concentric to make a big difference.
We can talk all day long about how to fix a compression chamber that’s out-of-round, but it is much more difficult than it sounds. The main problem is getting the air tube aligned with the axis of the cutter, so the cutting tool will cut in a straight line. That isn’t as easy as it sounds.
My best advice
The best thing I can tell you about this is to hold onto a spring gun that performs well. Maybe that would be a gun you tune and are amazed by how well it responds to the things you do. Remember the RWS Diana 45 I tuned in 2015? That gun really came out sweet! On the other hand I have tuned FWB 124s that simply would not go past a certain velocity limit that was disappointingly below 800 f.p.s. In fact the 124s seem to have this problem a lot. After listening to professional tuners, I now know those guns had compression chambers that were out of round. You can hone them until you are blue in the face — they will not go past that certain point.
The Diana 45 piston had a very fresh leather seal, but the improvements from a tune were impressive.
Out of round pistons
Let’s start looking at pistons and their seals. Since we have looked at out-of-round compression chambers, let’s look at a piston that was out of round. Remember the BSA Meteor Mark IV I tuned several years ago?
This piston in the BSA Meteor Mark IV was formed from sheet metal that was rolled and then welded into a cylinder. This one had broken welds at both ends, making the piston out of round.
After welding the piston back into a sort of cylinder the out-of-round problem went away.
Okay, I have been hiding the point, which is, as long as it isn’t too gross, an out-of-round piston doesn’t matter, because the piston seal itself is round and centers the piston in the compression chamber. In other words, don’t worry too much about the concentricity of the piston. It’s the seal that really matters.
Before we move on, I would like to point out that the “seal” in the Meteor isn’t what you expect. It’s an o-ring backed by a thin ring of another synthetic material. The front of that piston is steel and that is what contacts the end of the compression chamber at the end of the stroke. HOWEVER, it does not (or should not) hit the end of the chamber when the gun fires. When the piston gets to within a few hundredths of an inch of the end of the chamber, it stops on a cushion of highly compressed air. As the pellet starts moving down the bore the air pressure decreases and this cushion is squashed down. The piston then comes to a soft landing. That assumes the rifle is tuned correctly. All of this happens in thousandths of a second so we don’t perceive it, but the lack of damage on the piston end and the corresponding part of the compression chamber prove it.
Have you ever heard of a plain seal? Let’s look at one now. This one came from the “Steel Dreams” rifle that tried to break the sound barrier with a .22 caliber pellet.
This piston seal from the Steel Dreams rifle was plain. And it got burned during operation, which you can see as that flat spot in the center of the picture.
What this shows is it doesn’t take a lot of technology to push a pellet. This seal is as simple as can be, yet this rifle was as powerful as a tuned Beeman R1. That said, it was supposed to be much more powerful, as it was 125% larger and heavier and took 53 pounds of effort to cock. What that proved was it isn’t just the mainspring, the weight or diameter of the piston or anything else that many new airgunners think will make a spring gun shoot faster. It is the correct combination of all of those things, and can actually be realized in a smaller, lighter rifle.
Leather piston seal assemblies
Let’s now look at the old standby leather piston seal. These can be ever-so-simple, or they can be made as an assembly. Let’s look at one now.
Some leather piston seals are actually assemblies, comprised of several parts.
Instead of just a seal, this is an assembly of parts that comprises a leather piston seal.
This seal was made for a modern Chinese airgun. RidgeRunner can tell you that the leather seal in his vintage BSA 1906 rifle is simpler than this. It probably looked like some wadded-up old gym shorts that hadn’t been cleaned for a month when he first saw it. I have seen many of these old seals all squashed up and out of shape. Yet when you shoot the gun they still work — as long as they are well-oiled.
Okay, Arvizu, the parachute seal is an advance in piston seal technology. The first ones I know of date from the early 1950s. Anschütz and Falke put them in their rifles. The Hakim from 1954 has an example of one.
The Anschütz-built Hakim had an early parachute seal.
Parachute seals have become the standard today — so much so that I am surprised when I see anything else. But there are other seal types. Let’s look at a strange one now.
The PTFE seal
Polytetraflouroethylene or PTFE is commonly called Teflon in the United States. Teflon is a product of Chemours, a 2015 spinoff of DuPont, which discovered the product in 1938. What you need to know is that PTFE squashes when hit, and that is how the piston seal is sized to the compression chamber. There are several examples of airguns that have PTFE seals, but the Beeman P1 pistol is perhaps the best-known.
The Beeman piston seal is a plain seal that squashes into shape when dry-fired.
The process of fitting this seal to the pistol is to dry-fire the gun several times. The PTFE squashes out to fit the compression chamber in just a few shots. If your P1 still detonates when you shoot the cure is to dry-fire it a couple times. Works like a charm!
Webley also installed PTFE seals in their rifles years ago. I remember that my Beeman C1 had one. That seal broke in over time and just got smoother and smoother.
Now, the PTFE seal isn’t to increase the power, though the P1 is a powerful pistol. It is more about fitting the compression chamber better and also lowering the friction.
Last subject — seal materials
One problem with piston seals over the years has been the materials from which they are made. In the 1970s several companies had piston seals made of synthetic material that did not last well over time. Feinwerkbau, Diana and Walther all had problems with seal materials that degraded over time. Such seals can fail inside a gun with no warning. In recent decades guns made during this time by these manufacturers have failed and had to be resealed. The good news is the new seal materials show no sign of similar degradation.
A fresh 124 on the left. In the middle is the same seal that’s been in a gun for many years. This seal has been ruined by dry rot. On the right is an aftermarket replacement seal for the 124.
We started this report with a piston that was sealed with an o-ring. Now let’s look at a different type of seal that’s similar to an o-ring but very different from anything we have seen. This is the metal piston ring.
Like the piston in a car engine, an airgun piston can also be sealed with a metal piston ring. We know that the front of the piston doesn’t have to be protected because it doesn’t slam into the end of the compression chamber, so the metal seal works well even though the piston will rest against the end of the chamber. A car piston never touches the top of its compression chamber, but an airgun piston does every time!
You can see the split metal piston ring on this 1935 Webley Senior piston that has been lubricated, front and rear.
That is my discussion of piston seals. Can they increase performance? Certainly, however only within the limits of the compression chamber in which they operate. Will there be improvements in them in the future? Undoubtedly. But we are at the place where the improvements in performance potential are down to gains of just a few percent.