Yesterday I attempted to replace the piston seal on my Hatsan Striker Vortex due to a recent 75 fps loss in velocity. The stock piston seal had a pretty tight fit in the rear of the compression chamber as I was taking it out. I attempted to replace the Hatsan seal with a Maccari seal, but once fitted onto the piston, I could not get it to fit into the compression chamber - the seal was just too large. After getting it into the chamber a short distance I saw that it was not going to work and had to remove it, damaging the seal in the process.
Any suggestions on what I should do? Can a piston seal be resized somehow to fit into the chamber on my particular rifle?
your Hatsan I wouldn't doubt that your cylinder isn't truly concentric in the first place. Also most all seals need to be custom fitted to your rifle. There are experts here who fit them, then test them at holding pressure and vacuum test. A high quality rifle will have a perfectly round cylinder, a given stroke length, a given diameter of piston, piston weight, many variable piston seals. Pistons can have stick on buttons to center it. They can have holes counter sunk at various depths to center piston and buttons put on permanently. Some pistons have grooves milled in them for actual engine rings for compression. These are also cross hatch sanded by a Sunnen Honing machine which allows a geometric perfect pattern of slight grooves which hold lubricant. Wikipedia describes any of what I've written better than I. As does the search engine here on yellow. Just put in the words "fitting a piston seal." with no parenthesis or period. You'll find a WEALTH of information.
I learn by reading, watching, and mainly reverse engineering. I'm about to build a spring compressor and a cheap sprig gun to practice on. I want another Ruger Air Magnum. I'd actually like 2. When I owned them they only came in .177 and needed 10.5 Crosmans or anything lighter was over 1100fps. This factory rifle that's considered over sprung and junk by German Spring gun snobs could put Crosman 10.5's covered by a Quarter at 50 yards all day. I can only imagine what an actual well tuned one could do. It's almost an exact copy of an RWS 350 or a B-28 yet I've never seen either as powerful right out of the box. Ruger Air Magnums rock. I hear the Hatsans are very powerful also.
So the good news is yes your seal needs to be hand fitted with care and depending on how much material, what grit paper to use, or fine file. Bad news is your cylinder may not be true and you may need an entire new seal.
Good luck. If all else fails call John Thomas later today, after 4pm and before 9pm at 1-717-873-5128 as he is VERY good and fairly priced. Well worth EVERY cent spent. A true professional.