How to replace an airgun breech seal

Posted on 30/08/2020 by towens — No Comments ↓

Simon Everett shows how easy it is to replace a breech seal, even a vintage leather item, breathing new life into a much-cherished rifle.

Replacing the breech seal on a vintage rifle like this Cadet only requires a few basic tools – but the job is far easier if you have a means to hold it stable

Leather has been used as a sealing material in all sorts of applications ever since the beginnings of engineering. Leather is a remarkable material. It has slight elastic qualities, is hard wearing, flexible and is also easily worked into required shapes.

It was these properties of leather that permitted the gunsmiths of the continent to make the very first pressurised airguns as far back as the late 1500s, and there is a bellows airgun from 1580 in the Livrustkammaren museum in Stockholm.

Leather provided the means for gunsmiths to create a seal that would be tight enough to make an airgun work effectively – and also to fill the bellows to fire the forges that would allow them to heat the metal to be worked at high temperatures.

Leather seals for both the piston and the breech were still in common use for airguns up until the 1970s when synthetic materials took over. There are many airguns still in use that use leather for their seals, such as this BSA Cadet from 1947.

The old seal had perished after years of use and was no longer making anything like a reliable airtight seal

As you can see, the original part had shrunk and was not making any kind of seal whatsoever. Shooting over the chrono gave a variable result between 2.6 and 3.4 foot pounds.

Over the years a leather seal will wear out, dry out and crack unless it’s treated with an inert oil. Any seal, leather or synthetic, will need renewing, as the repeated expansion and compression from opening and closing the barrel gradually wears out the seal.

Fortunately, changing the seal is a fairly straightforward job that the majority of airgun shooters can tackle themselves with the most basic of tools.

It makes sense to get everything together before starting. You will need a screwdriver and something to help wheedle out the old seal. I used a bearing seal picker and snipe-nosed pliers.

You’ll also need some inert oil. Neatsfoot oil is my preference for leather, but something like Parker-Hale Express Gun Oil does work, and is what I had to hand. A clean rag is a good idea to wipe surfaces as well as your hands. And finally, you’ll need a suitable replacement seal.

It isn’t strictly necessary to have a bench vice, but it does help greatly by freeing up your hands so you can work more accurately, safely and efficiently. My vice is portable, and it was perfectly adequate just sitting on a garden table allowing me to work outside in the sunshine.

I made some soft jaws from folded corrugated cardboard that I cut from a box, in order to protect both the wood and metal parts of the rifle whilst gripped gently in the vice. Be careful not to overtighten the vice and crack the stock.

When taking off any type of gun stock, identify the screws you’ll need to remove and the tools you’ll require to do it

This stock on this particular rifle is held in place by two screws in the forend and one at the base of the trigger guard

With the stock screws removed the woodwork and metalwork can then be parted – who knows when this Cadet was last seen naked?

Being a rifle of relatively low power, no spring compressor is needed and the end cap can be unscrewed by hand

The spring and guide can now be removed, making the rifle completely inert and totally safe to work on

Simon’s next task was to clamp the action at the cocking block in order to hold the barrel still

A bearing seal picker is the ideal tool to use to dig out the old seal as it has the required degree of finesse

What’s left of the old leather washer can then be carefully pulled out with a pair of snipe-nosed pliers

Replacement seals can be made from a variety of suitable materials, but Simon chose leather to retain the integrity of this 1947 classic

Simon cleaned out any remaining detritus from the seal groove with a wooden cocktail stick before fitting the new seal

He pressed the new seal into place with his fingers, carefully working around the edge to maintain the correct shape

The leather seal was then gently tapped fully home, with Simon using the handle of his screwdriver rather than a mallet

At this stage the new seal was standing proud and had to be carefully trimmed down

Simon repeatedly closed the barrel to check the fit, trimming off a little more leather if the seal was still standing too proud

The washer itself showed Simon where he needed to trim, taking care to cut away just a little at a time

Simon put a couple of drops of oil onto the leather seal to swell it, but only after it had been properly seated and trimmed to fit

With the rifle held snugly, the first job is to separate the wooden stock from the metal workings. Using a properly fitting screwdriver, undo the stock mounting screws, typically two at the forend and one at the rear of the trigger guard.

That is all that holds the gun into the stock, so be ready for the cylinder and barrel to come loose and possibly fall out. I used an old ice cream tub to hold the parts as I removed them.

On these early BSAs, dismantling is a very simple task. The sloped end cap simply unscrews by hand, there is very little preload on the spring, so whilst there is some pressure from it when the final thread is released, it isn’t strong enough to worry about provided you are prepared for it. With the end cap unscrewed, the spring and spring guide can be lifted out entirely. Place them carefully in the tub together with the end cap.

Now there is no longer any tension on the barrel, you can reclamp the gun by the cocking lever and the attachment for it below the barrel. This will allow you to work on picking out the old seal with the barrel held firmly.

I used the bearing seal picker to carefully dig out parts of the old seal until enough material had been lifted out of the groove so it could be gripped
with my snipe-nosed pliers and pulled free. I then cleaned out the seating groove around with a wooden cocktail stick and blew off any residual bits of the old seal.

Now comes the interesting part – fitting the new seal. I had a small selection of seals of various sizes to choose from. I wanted to stick with leather to keep the Cadet as original as possible, so I offered the two I had up and went with the smaller one as it looked the closest to the dimensions of the barrel and seal groove.

With the internal circumference of the leather seal placed carefully around the barrel, I pressed the seal with my fingers to start working it into the groove. It is very important to do this task while the seal is still dry.

Then I gently tapped the seal with the handle of my screwdriver to knock it home, bit by bit, continually reshaping the leather as I went. It was a case of a bit of tapping, a bit of shaping, then a bit more tapping and a bit more shaping. Gradually the seal worked its way into the depth of the groove. I have never understood why the breech seal groove is so deep, unless it allows extra elasticity and resilience within the seal.

With the new seal tapped fully home, it was way too proud to allow the barrel to close, even though the top of it had been compressed. So it needed a bit of shaping to finish it off, and this was done by trial and error, shaping it and trimming it little by little. It is not a precise science.

Using a very sharp knife, I removed a slice across the top and then closed the barrel to check for fit. Gradually, sliver by sliver, I trimmed the seal to fit. Some people advocate using a large washer that fits over the seal to act as a cutting guide and I have no doubt this was probably the method used in the factory to speed up the process and produce a uniform-fitting seal, but I had to go by eye and just take my time.

Once the barrel would close properly without too tight a fit, I put a drop of oil on the seal, and with the barrel open allowed it to soak overnight. The following day the seal had completely soaked up the oil, so I put another drop on the opposite side and left it to soak for another 12 hours.

At the end of this time I had a seal that looked pretty good. It was a little bit proud of the breech and the barrel closed snugly each time. Putting the rifle over the chrono it was now producing between 5.8 and 6.2 ft-lb and hopefully will give a few more years of fun. Job done!