Jim turns his attention to his old HW55, and its conversion from a leather to synthetic piston seal
Before the introduction of the synthetic piston seal, the first of which appeared in the late 1970s Feinwerkbau Sport, leather was the material of choice, and had been for almost a century. In many ways, leather is an excellent material for piston seals, not least because one can be fashioned by the DIY inclined from recycled leather from shoes, belts or furniture, but leather has a drawback insofar as it has to be ‘wet’ lubricated.
For many years, leather seals were lubricated using neatsfoot oil, an animal fat extracted from the shin and foot bones of cattle, and which is unusual for an animal fat in remaining liquid at low temperatures. In some respects, neatsfoot oil is excellent for seal lubrication, because it softens the leather to allow it to conform to the cylinder, and conditions it, although on the flip side, it oxidises in time so the leather hardens, and it auto-ignites at 442C. As we know all too well, autoignition is associated with dieseling.
Neatsfoot oil slowly gave way to various light mineral oils, which also auto-ignited, some at lower temperatures than neatsfoot oil. As the late Mike Wright established, auto-ignition creates a pressure spike in the compression stroke that can get the pellet moving earlier, increasing the time that the piston and pellet are travelling in the same direction, increasing muzzle energy, and many spring airguns with short piston strokes, including the HW55, could only produce reasonable muzzle energy by harnessing auto-iginition. New airguns were delivered with a plentiful supply of oil to ensure prolonged dieseling, and advised that a couple of drops of oil be applied through the transfer port at regular intervals.
Through decades of trial and error, airgun manufacturers had established that their diesel-dependent guns produced maximum muzzle energy with wide diameter (typically 4mm) transfer ports, although for a variety of reasons, when they started to supply guns with synthetic piston seals, they discovered that the guns gave their best with far narrower transfer ports, typically from 2.9mm to 3.2mm. Again from Mike Wright, we know that the larger diameter transfer ports of dieseling airguns delay piston bounce, maximising the ‘cylinder pulse’ – the piston travel between pellet start
and piston bounce – and the energy given to the pellet.
More recent airguns with synthetic piston seals tend to have longer piston strokes, so don’t need to diesel, nor do they need to delay piston bounce by having a large diameter transfer port. In fact, if anything, they need narrower transfer ports to advance the onset of transfer port, mass air flow choking, because that’s when the air’s internal kinetic energy is converted to ‘flow’ energy. So, to gain the most from converting a leather sealed airgun to synthetic, the 4mm diameter transfer port usually needs to be sleeved down to nearer 3mm.
The HW55’s contemporaries and stablemates the HW50 and especially the HW35, which sold in huge numbers, are candidates for transfer port sleeving, as well as the Anschutz 335 and others, if switching from a leather piston seal.
THE BASIC CONVERSION
Most leather piston seals are attached by a central fibre washer, itself held to the piston face by a machine screw running into a threaded hole in the piston. The conversion entails simply removing the leather seal, and fitting an adaptor using the original machine screw, then the new seal.
The most popular adaptor is the one that allows Weihrauch seals to be used, which also gives access to various aftermarket seals designed to fit Weihrauch airguns. If you have access to a lathe and an offcut of Acetal or similar engineering plastic rod, an adaptor can be turned in a few minutes at a cost of pennies; if not, expect to pay upwards of £10 for a commercially made one.
Many people who have converted old spring airguns from leather to synthetic seals have reported a fall in muzzle energy as a result. To a small extent, this is due to the already short stroke being made a millimetre or two shorter by the seal and adaptor, but the main reason for the fall in muzzle energy is the reduction in the cylinder pulse due to the lack of dieseling.
The 4mm diameter transfer ports of many airguns with leather seals may delay piston bounce slightly, extending the cylinder pulse, but this alone is not enough to compensate for the loss of dieseling and consequent early pellet start. The loss of muzzle energy can be mitigated to an extent, though, by advancing the onset of transfer port choking through sleeving the transfer port.
Some years ago, I used my old HW77 and a range of transfer port inserts to prove something, probably for the first time, that many had long suspected, which is that there is a general relationship between transfer port length and energy efficiency, with shorter ports tending to be more energy efficient. Airgun
manufacturers may have suspected the same, but produced airguns with ports in excess of an inch long regardless, probably due to potential problems sealing the cylinder to the end plug with less plug length inside the cylinder, as well as concerns about the strength of the joint. The HW55 transfer port is not untypical at 25.9mm long.
Shortening the HW55 transfer port is not practical because it would entail removing the end plug/breech block, machining and refitting it, and that would leave a short length of internal thread on the cylinder wall for the piston seal to rub against. Other rifles may be different, but with the HW55, I’m stuck with 25.9mm.
I had previously made and fitted an Acetal transfer port sleeve with a 3mm hole, but later removed it after it crept forward a fraction into the breech, so decided to try a more substantial alternative.
The sleeve needs to be 4mm external diameter with a length of 25.9mm and a central hole ideally of 2.9mm to 3mm. Unless you have a model engineer’s lathe, then due to the potential for flex caused by the length and diameter of the insert, it would perhaps be better to start with a length of 4mm brass or perhaps mild steel rod, than to machine a larger diameter rod down.
THE AERIAL ROUTE
My machine is way too big and cumbersome for such precision work, although I’m sure a skilled machinist could make it work, and a half remembered post on AirgunBBS gave a cheap alternative that did not need a lathe at all, so Jon Budd, please take a bow. Jon’s solution was to cut a length from an old 4mm diameter radio aerial, a positively classic example of ‘shed tuner’ ingenuity!
As luck would have it, the aerial of my workshop radio had broken off recently, so I retrieved it from the waste bin, and found that one section was 4mm in diameter. The internal diameter of 3.5mm was larger than ideal, but a step in the right direction, so I cut off a 25.9mm length using a fine cutting disc in a high speed drill, cleaned up the ends with a drill bit, and slid it into the HW55’s transfer port with a tiny dab of locking solution to hold on to it. Job done – cost £0.00.
My target was the HW55’s designed muzzle energy of 7.5 joules (6ft.lb.), which it achieved easily. I have previously achieved far higher energy from the rifle, although at the cost of losing the sweet-shooting characteristics of the original. When time permits, I’ll almost certainly obtain a length of 4mm brass rod and replace the length of aerial. It and many classics of the late 1950s to 1970s are more than worth the effort.
The ideal port sleeve would be 4mm tube with a 3mm bore, but even sleeving to 3.5mm is a step in the right direction.
Another alternative to a leather seal is to turn a synthetic cup seal like this, which is for an Anschutz 335.
Even after molybdenum lubricants were widely adopted, lavish applications to leather piston seals encouraged dieseling.
With the surplus grease removed, it’s plain that this seal has long passed its ‘sell by’ date!
With around 3mm of recoil, the HW55 is a joy to shoot.
Tip: A couple of blocks of Acetal in the cocking link prevent galling with the cylinder.
A sharp twist bit cleaned up the cut edges perfectly.
The sleeve ended up a fraction shy of 25.9mm, but that did not seem a problem. I put a tiny dab of locking solution on the end of the sleeve before pushing it home.