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PostPosted: Sun Oct 06, 2019 6:51 pm 
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Joined: Thu May 04, 2017 6:41 pm
Posts: 133
Location: Vancouver BC
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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PostPosted: Sun Oct 06, 2019 7:17 pm 
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Location: Klowntown BC
ahhh... opted for Good, Cheap.
good choice(s).


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PostPosted: Mon Oct 07, 2019 10:03 am 
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Location: nvvan
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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PostPosted: Mon Oct 07, 2019 2:34 pm 
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Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1255
Location: United States
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.


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PostPosted: Tue Oct 08, 2019 7:30 pm 
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Location: Vancouver BC
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.

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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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