Spring piston seals
by B.B. Pelletier
Yesterday, TC told me he wasn't familiar with what a piston seal looks like. I linked him to the 13-part "Spring gun tune" report, but nothing in that report really explains a piston seal, so today I thought I'd do that.
The first piston seals were simple, flat, leather pads screwed to the end of a steel piston. They worked but weren't too efficient. Around the turn of the 20th century, the cupped leather seal was born. The cupped end faces forward and fills with air as the piston rushes forward. The air inflates the leather cup, driving its sides against the compression chamber wall. That gives a better seal than a flat leather pad.
In the middle of the century, there was a brief use of metal piston rings as seals. They work exactly as they do in a car, sealing the compression chamber by expanding into the wall. They had several drawbacks. First, they are precision metal parts and cost a lot to make. Second, because of their location, the volume ahead of the piston is not as small as it would be if the seal were located on top of the piston instead of around the side. For this reason, piston rings are reserved for low-velocity airguns.
Piston from pre-1940 Webley Senior pistol has a beryllium-copper piston seal that looks like a automotive piston ring. When the piston is inside the gun, the ends of the seal are pressed closed. Pardon the grease, but I'm not about to relubricate my pistol for anyone. It will still work fine 25 years from now.
Teflon is a synthetic with a low coefficient of friction. Also, it squashes and holds its shape, so companies like Weihrauch and Webley have experimented with it as a piston seal. It works well in the HW 45/Beeman P1. You fit this seal by dry-firing the gun several times. It simply squashes to fit the cylinder. No lip is possible because of the squashing nature of the material.
The Beeman P1 has a Teflon piston seal. This is the top or front - the side that faces the air transfer port. No lips are required because Teflon squashes to fit the compression chamber.
The synthetic parachute seal is best seal of all. It can be fitted to the compression chamber so it drags very little; but, when it goes forward, the lip of the seal inflates and expands against the compression chamber wall just like the old leather cup.
There are many variations on the shape of the parachute seal, but they all do the same thing - expand against the compression chamber wall to seal the air in front of the piston. When you oil the piston, you help the seal do its job, because the oil fills in all the microscopic scratches on the piston wall. But, the oil also burns, so over-oiling causes detonations that can ruin the piston seal and mainspring.
This is a Beeman Laser piston seal that was part of the laser tune they sold for the R1. It's a classic parachute seal because of the lip that blows out into the compression chamber wall. This one's been fitted to my rifle. Because of the synthetic material in the seal, a special grease called Laser Lube was used with this tune. The round impression in the seal is from the air transfer port, made by the piston slamming into it several thousand times.
Vortek experimented with several R1 piston seals in the 1990s. I tested this one along with several others. It doesn't have a typical parachute lip, but the flange around the top serves the same purpose. Note the V section on top was perfectly centered on the air transfer port.
The famed RWS Diana blue synthetic seal is one of the longest-wearing and most self-lubricating piston seals on the market. They often need fitting to the gun by thinning the sides of the parachute lip. They also require very little lubrication.
These two Chinese piston seals from the 1990s are examples of "Monkey see, monkey do." The Chinese copied the parachute design without understanding it. The result was these colorful seals made from synthetic ticky-tacky that were totally inappropriate for the job intended. They don't do well in airguns, but they look good enough to eat! The Chinese have since learned their lesson in seal-making.
The legendary FWB 124 was one of the first airguns to use synthetic parachute piston seals. Unfortunately, they used the wrong formulation material, which disintegrates over time. These are totally useless, having turned to the consistency of hard wax over the years. Modern replacement seals for 124s are made of much better stuff.