Replacing a leaky piston seal with Jonathan Young

A leaky piston means a replacement seal, and with an older rifle that means leather, so Jonathan Young explains how to make one the old-fashioned way

Older airguns are great fun to use, and let’s not forget that these were put to serious use in their time. Many were capable of outputting very good power levels, and guns like a pre-War Improved Model D can reach 11 ft-lb with a tune today. Most of these older guns will have piston seals made of leather unless they’ve been subjected to a recent modification by a keen home tuner.

Picking one up and testing it may show muzzle velocity is down, not that this in itself is a bad thing. 

There will be less recoil for a start, so accuracy should be improved. But there comes a time when even the best of the best needs looking at. There is nothing wrong in using modern materials in older airguns, but for this article I’m sticking to the original seal design, and that means using leather.

Fitting a replacement seal to an older spring piston airgun is a task that’s simple to do, but it’s easy to get it wrong, leading to hours of head-scratching, so it pays to know exactly what you’re trying to reproduce.

Inside many recoiling air rifles, especially really old ones, there may be a piston seal made from a single thick piece of leather, possibly with a thinner leather washer used as a backing piece to gain height. 

Trying to find leather that thick today can be nearly impossible. As a remedy some thinner pieces of leather can be stacked together and glued. Pre-made industrial leather washers are ideal if some can be found in roughly the same diameter as the piston head. 

For a start they come in specific overall diameters and with a hole already punched in the centre. Trying to punch a hole symmetrically in the centre of a round object is actually very difficult, but leather punches can be bought cheaply for those who are confident and not cross-eyed.

Backing the main seal with another washer or spacer can compensate for any lack of height in the new replacement leather washer. Some machined plastic material formed into a disc can work too. It may not be original, but it can be just as effective.  

The height of any piston seal assembly will affect muzzle velocity somewhat. In theory the swept volume inside the area ahead of the piston seal that helps determine overall performance can be improved by having a seal that’s a bit less tall in the saddle. This decrease in mass should in theory give an automatic increase in the swept volume.

However, having a seal that is not up to the job and underperforms will result in a shorter life for the seal and possibly the airgun too. Dry-firing a springer is, as most shooters know, really very bad for any spring airgun, but a loose and badly fitting seal must surely also have some detrimental effect.

Fish oil and neatsfoot oil have traditionally been used to keep leather seals supple, but many tuners appear to favour man-made silicone oil these days. 

When soaked in a suitable oil, vegetable-tanned leather can expand, ready for fitting to deliver a perfect fit, and if any glues were used hopefully these will not be affected or dissolve. In the example here, cyanoacrylate glue was used, which seemed impervious to the fish oil used to soften the leather before it was inserted into the spring cylinder.

Getting the correct shape and fit can take time. Sometimes there is no option but to use a much larger diameter piece of leather, which can then be sanded back to size. Fitted to a long bolt with a nut, split-ring washer and a larger flat disc washer, the bolt can be tightened into a hand drill or drill stand. 

The leather can then be sanded, so long as you’re constantly checking the progress made. Frequent test-fitting should hopefully see the new seal end up making a perfect fit, which in turn will let the gun deliver optimum performance.

BSA air rifles and some other models, especially those from the 1930s, used a different seal design called a cup seal. A flat sheet of leather is wet-formed in a cup-shaped recess, leaving the leather to dry into – you guessed it – a cup shape. 

Another thick, flat leather washer – or a man-made nylon or steel spacer – is placed inside the cup to strengthen the outer edges. All parts have a centralised hole drilled for the retaining screw to fit the assembly onto the piston. There may even be a dished steel washer to support the screw as well.

When assembled together onto the piston, a robust and sturdy seal is created from all these parts. 

With the pressures involved inside the spring cylinder tube, most prefer to hand-tighten only the seal assembly screw to the piston, having smeared a little threadlock onto this screw first. Oddly it’s very common to find jammed screws in the threaded piston head. 

When these do finally come out, so too does the thread, and this is usually seen as a long curly spiral of steel. So that’s why dry-firing any springer is a very bad idea.

Using thick, hard leather for this job formed by an industrial press is not achievable for most home tuners, but a lot can be done with some leather offcuts, some suitable cup-shaped containers to make a mould and a simple bench vice to supply the pressure needed to form the leather. 

A mould can even be made by drilling out a piece of wood or plastic to roughly the correct diameter. The leather, which has been soaked previously in water, is then laid over the mould and a former is then pressed in over this. 

Inserting all this into a vice and leaving it overnight under pressure should reveal in the morning what looks like a rough cupcake paper casing, which will be the basis for a new seal after it’s been trimmed and dried out. Untreated veg-tanned or tooling leather is needed here as this absorbs water, which helps soften it, and this type of leather will retain its new shape when dry, when most other treated leathers do not soften and cannot be formed into shape.

One option is to use softer, thinner leather for the main cup-shaped portion. This is counter-intuitive when, with all the pressure and bashing around inside, one would expect any seal would need to be robust, and this  has appeared on less powerful airguns. 

The edge of the leather seal expands and hugs the cylinder wall when the piston slams forward. In theory this idea should work on any airgun, and indeed some seal kits bought online use this much softer leather along with a stiff man-made central washer.

Experimenting is a big part of fixing, and people have even written books on the subject. So beware, spring gun tuning can become addictive!