@Crambo ,
I had difficulty measuring the diameter of the seal of course because it's a soft polymer, so I attempted to use a feather light touch with my dial calipers as a relative measure of how my sanding process was going in between test fits.
I sanded my seal by chucking the piston and seal together into my hand drill by sticking the rear shank of the latch rod into the chuck. That should give you a good picture of a simple successful method. The seal flexes when you put pressure on it and sand it of course, so I tried to concentrate more of the pressure on the rear end of the seal, helping to preserve its tapered shape and forward leading edge.
What's the worst that can happen? You need another seal. Easy enough to remedy. Take your time.
Every so often I would stop and do a test fit in my cylinder with a tiny amount of molybdenum disulphide grease. That means carefully tucking the lip of the seal past the cutouts in the rear of the receiver as you ease it forward. I didn't measure the force needed to push the piston but it was certainly strong enough to hold itself vertically in place yet still move with some pressure from one finger. I don't have a final caliper measurement because I felt that an absolute measurement would be unreliable, and different, as fitted, for each cylinder. A relative subjective fit test seemed more appropriate. I guess if you wanted to put science to it you could use a small fish scale to measure the force needed to pull the piston back out of the cylinder. If I was a tuner, I would probably have a repeatable setup like this with super secret notes as to what amount of friction was suitable for what rifle and seal, etc etc. That's just the engineer in me brainstorming.
The parachute types of seals, if you look carefully from the front, there is an annular groove near the outside edge. As the piston makes pressure, the pressure expands the lip of the seal during the shot cycle, increasing the sealing ability. It is sometimes called a parachute seal for this reason if I have my terminology correct. That is why it does not have to fit terribly tightly during a static fit test.
I am not a commercial tuner, and I have only done the seal on my 124 one time so all I can pass along is my experience from one repair on this gun. I've put seals in some of my other spring guns with a similar process.
You've come this far, you already have the skills for disassembly and hopefully a compressor for reassembly so you can do this.
I manufactured a simple cylinder swab using an oak dowel with a kerf cut into the end into which I could insert and spiral wrap any number of bits of rag, paper towel, or Scotch-Brite with 99% rubbing alcohol as a cleaner/solvent. You could even go as far as to buy a cylinder hone I suppose, but I felt that was not necessary for a basic clean out and Lube tune, especially if an inspection of the interior did not reveal any irregularities or damage.
I have read that some tuners do this and move the hone in and out as it spins to create tiny helical scratches that retain some lubricant but I don't have enough experience to know if that's even worth it.
I used the same kerfed dowel described above with a paper towel, chucked into my hand drill, to very lightly burnish molybdenum disulfide grease into the inside of the compression chamber portion of the receiver tube. That's where you want the molybdenum disulfide bonded to the bare metal for long-lasting results without much grease buildup that would otherwise cause too much dieseling upon break-in.
Shine a flashlight down into that cylinder to look for smashed up bits of old piston seal that may have compacted into the end of the compression chamber. I'm trying to remember what tool I used to attempt to scrape any residue. You could probably use another, fatter oak dowel and make an elementary rotary scraping tool by saw cutting some kerfs across the dowel face, shoving it into the receiver, pressing and rotating. You could also just make a simple wide kerf and cut a piece of 1/8 in plastic and glue it into the curve to make a hand-operated, rotary face scraper tool. These are two examples I can think of that would be a safe tool that anyone could make at home and would not damage your receiver.
Best of luck with your repairs, let us know how it turns out.
Feinwerk
