Canadian Airgun Forum

Got aa Hatsan mod 55s Craigslist buy that has never shot well, shotgun groups and a piston that slams so hard the scope block screw sheared off. I beefed up the screw and this gun de-focused a Springer rated Bug Buster scope. I fixed the scope but it still blew my mind because the front objective was seriously cemented in with black hard goo. Yet this gun moved it out of focus.


Thanks to forum member Kelly I looked into the hatsan pistons and several posts on the Euro and Australian forums has ppl drilling or grinding some weight off the piston. Additional forum posts claimed Hatsan piston seals are under-sized and sure enough mine goes in the tube with no problem. So to counter the under-sized seal I'll use thick moly paste instead of silicone oil to lube the seal.

Anyway in all my searching I could not find a photo of a lightened piston. Just descriptions how to do it. So here's my result. Three slots along the piston body , top and sides about 3/8" wide. The grinding took some time and I cooled it with frequent douses of water. the filling,smoothing and sanding the rough edges was more of a chore. No way my $100 drill press could have touched this steel so grind I did.

I also retapped the scope block screw from 6/32 to 10/32 and used a hex bolt from a bicycle water bottle mount.

Yes, this is the gun from the other post; I cracked the stock.
Update to follow...

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Good, Cheap, Fast; pick two.

ahhh... opted for Good, Cheap.
good choice(s).

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I would think they slam hard becuase they are sealing well. Any chance to fab a tighter seal, which could slow the piston down at end stroke?

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That piston impact and associated accuracy and power loss is due to leakage, as a general rule.
So I'd find a seal that fits correctly. No need to lighten the piston, which probably won't hurt it but I doubt it'll help.
From what you described I'd assume the leakage is the main seal. They need to be snug so air can get past, then when psi builds it'll expand the edge of the seal to match the sealing force needed. If it leaks a little then it can't expand until psi rises much further, meanwhile you're losing air. It's the air that slows the piston down to minimize impact, and of course launch the pellet, so both suffer is there's any leakage and the more you leak the worse it is. Also check the breech seal as well. The thicker lube may help, for a bit, but if it does it won't last.
If you can't find a seal specifically for that gun you can post the dimensions along with pix and I/we can probably come up with one for another gun that'll work. We'd need the OD + the large and small IDs, plus closeup of the hole in it and the part of the piston it attaches to.

You are right. Lightening the Piston had little effect.
Moly makes the velocity consistent. I get a standard deviation of 4.9 with the moly. With silicone oil the standard deviation was all over the map from 24 at the widest to 15. This is after all big bang dieseling shots from fresh oil were worked out.

Buy you say the moly won't last

There is a new factory seal on the piston.

After 100 shots the stop block screw was tilted but seems to be holding poi.



There is a seal available from customairseals.com

Now I did up a leather shock absorber but I'm out of pellets to test with so once again the nice looking terrible shooting Hatsan 55 goes back in the closet.

I'm thinking the sub500 spring can not move fast enough to parachute the seal ( to use a term I read on the Australian forum). And the slow firing sequence makes this gun super hold sensitive.

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Good, Cheap, Fast; pick two.

Well, a new seal only works if it's installed w/o damaging it. The seals are usually all trashed at the factory b/c the gun has sharp edges inside that cut it up. So unless you round those edges you likely trashed the new seal. If you installed the same seal a second time you likely made it worse that it was.
A weaker spring does not make it more hold sensitive, as a general rule. Meaning a particular spring could make a particular gun worse b/c of the shot cycle and barrel movement throwing the shot, but on avg a lighter spring makes it less sensitive and stronger ones make it worse. This has to do with the piston impact, which I believe you have an excessive amount of thanks to a bad seal. A poor gun or seal design can do it too, and since I don't know the Hatsan 55 I can't say.
Why don't you email me at chevota at hotmail so I can send you my tuning guide. It'll explain a ton of stuff that I imagine you have questions on. Including stuff like; how to make a scope mount that won't slide back, even if you have a problem gun like yours Of course fixing the source of the prob is better, and will no doubt net more power and accuracy, but it's an option.