Thanks to Rob M for the following advice :-
Making leather piston washers
Hints on making a new leather piston washer:
I get off-cuts of sole leather from a cobbler's shop to make new piston washers.
It's good to make your own washers, with a few scraps of sole-leather you have all you need to keep an old-fashioned springer working for a lifetime! It's also satisfying to make the bits you need to keep your old guns alive. These methods can help with the manufacture of leather breech seals too if you should need them.
The ways that leather washers are affixed to the piston vary from manufacturer to manufacturer. Some are held on by bolts, hex bolts or machine screws, others are riveted on.
To make the washer, measure the diameter of the old washer (or the piston) then add twice the height of the washer to the diameter. Cut out a disc this size (approximately) from your leather, carefully use a sharp knife or fretsaw or similar. Find the centre of the leather disc, and drill a small hole there, then, with a nut and washers, fasten the leather disc to a bolt that you can hold in the chuck of an electric drill. Now you can use the drill as a lathe, and use some clean files and wet and dry abrasive paper to round the disc off perfectly and also make the leather thinner if necessary. It has to be "Wet and Dry" type abrasive, as sandpaper or glasspaper will shed bits of abrasive that might end up embedded in the leather, this doesn't happen with "wet and dry". If using a drill or lathe wear eye protection. Depending on how much reshaping you need to do, a dust mask might also be necessary. You can shape leather like this this easily by hand if you don't have power tools, but it will take a little longer.
If the leather has a shiny side, I usually remove the shine with Wet and Dry, as it is sometimes a paint-like finish applied to the leather and could have adverse effects on the eficiency of the finished washer. Some say that the shiny side should go on the outside of the washer where it contacts the cylinder wall, but washers made of raw unshiny leather soon develop a shine after a few dozen shots anyway.
Most leather washers are actually a pair of washers,a large rear washer that is formed into a cup shape, then a smaller front washer that fits inside the cup to minimise lost volume, so you will probably end up making two discs, one smaller than the other.
Fit the washer(s) to the piston, then fasten a jubilee clip around the washer to shape it into the necessary 'cup' shape. It's worth taking your time, tightening it up a little at a time over a period of a few hours. I spray the washer liberally with a little aerosol of stuff I got from a cobbler's shop called 'Leather Stretcher', which actually just softens leather enabling it to be formed into shape more easily. This stuff really helps, and makes the job much, much easier. Tighten up the jubilee clip until the washer fits easily into the cylinder. Spray it, retighten the jubilee clip until you are happy with the fit and shape, then leave it to dry and set overnight or for a day or two if you can. If necessary, any bits of leather that you feel need removing can be carefully trimmed off now with a sharp knife.
Don't worry if, at the moment, it does not look anything like the old washer: The last step in the procedure will shape it precisely to fit the piston and the cylinder. At this point, the washer might appear much thicker than the original, and have a large gap between the back of the washer and the piston, especially around the outside, but don't worry. The important thing now is that the piston can be refitted into the cylinder without the leather washer getting damaged on threads, cocking slots, etc.
Before refitting it, let some "Bisley Gun Lubricant", or "SM50" soak into it for a few hours, or overnight, this is a silicone oil that will not cause the leather to disintegrate over time, plus it contains molybdenum which will lubricate the leather in use. Let the Bisley Gun Lubricant soak well in, but before refitting it, you might need to dab it with a clean cloth or tissue if there is much silicone lube on the outside of the leather. Silicone lube is good for the leather, but no good where metals will be in contact with each other, so you don't want it dripping off the washer into the cylinder or smearing the cylinder wall, for instance.
Obviously, if you are going to put GN paste or moly grease on the piston, do that now. When you have finished reassembling the gun, let the gun stand for a few days with the piston at the end of the cylinder under spring pressure (as if it has been fired), so that the spring pressure can slowly form the washer to fit the cylinder perfectly. Avoid the temptation to fire it straight away! This last step shouldn't be rushed, as allowing the leather to gradually conform exactly to the shape of the cylinder, rather than slamming it suddenly into shape by firing the gun straight away, will increase its life and help to stop it disintegrating when it gets old.
If you do all this, and haven't made the washer too big or too small, you should have an efficient and long-lasting piston head washer... It may take a short while to bed in, but it shouldn't require any more lubricant for thousands of shots now. If you should ever want to relube it I recommend taking it out of the gun to do it, rather than putting silicone oil down the transfer port which could end up damaging the piston and the cylinder.
Even if you buy a pre-made washer, some of these tips should still be useful.
Hope this helps!