Need a Gamo Magnum IGT piston seal

[MontanaMarine]

Velocity with the Crosman 14.3gr HP dropped from 850-ish fps, down to around 612 fps.

The gun is only about 15 months old, and has between 500-1000 pellets through it (estimate).  I know it has a 5-year warranty, but I couldn't be bothered with jumping through all the administrative hoops.

So anyhoo, I took it all apart, and as I suspected, the piston seal is toast.  I've never dieseled it or anything, other than when it was brand new and breaking it in burning off the factory oil.  Even then I swabbed the bore clean before starting.

I had to fabricate a redneck spring compressor using a clamp, socket, and a pusher I made from conduit tubing.  It worked pretty slick.

Here's a couple pics,





I think the Custom Seals outfit in Australia makes the piston seal that I need.  If anyone else knows another source, please share that info.

Thanks, Shane

[MontanaMarine]

Found a piston seal on Ebay, should be along in a couple weeks or so.  Looks like it is coming from Spain.

[A.K.A. Tommy Boy]

Clocked in and following updates.

[Thor760]

I installed a seal for my Bone Collector from that same Australian place.  I would also recommend a good compression tube cleaning/hone while you are in there.  I did that and applied some Super Moly paste to the Piston seal and tube from Air Rifle Headquarters.  What a difference.  Smoothed everything out and cleaned up my standard deviation to around 1.5 FPS.

[longhunter]

What diameter is the compression tube?
Looks an awful lot like a Diana/RWS seal.

[Mark 611]

I think some of those bigger Gamos are a 30mm?

[Back_Roads]

 It does have the Mach1 powerplant so the 33mm seal is needed. I did a rebuild for a friend whose failed in about the same time period. https://www.customairseals.com/product/gamo-33mm-piston-seal/

[MontanaMarine]

Update,

I have the piston seal replaced, and the rifle is reassembled.

It took a while to get things I needed here.  We've been in an evacuation status recently for wildfire (Horse Gulch Fire), so no mail or UPS, Fedex runs for several days.  I was able to get to town and pick up some stuff the Post Office was holding, including a heat gun I needed to soften up that piston seal for installation. 


I should get it over the chrono soon.  When the gun was new I was getting around 850 fps with the Crosman 14.3gr, and around 760 fps with the JSB 18.13gr.  Those velocities are at 4000 ft elevation.  So, that's what I'll be using as a basis of comparison with the new seal.

Shane

[MontanaMarine]

Yes it does have the 33mm tube.  The inside diameter is actually 29mm. 

My Whisper Fusion Mach 1 also has the 33mm tube, as does the Bone Collector, but the Magnum tube is longer.  The Mach1 and the Bone Collector share the same powerplant.

[MontanaMarine]

Zero'd the optic and did some chrono shots this morning.  I only used the JSB 18.13gr.

Velocity averaged right around 768 fps (4000 ft el.), and accuracy at 25 yards was around 1/2", I did manage one group that was only 1/4".

I looked up my old notes.  Before the old seal gave out, it was getting around 775 fps with the 18.13gr.  So It is a tad slower, but maybe it will gain a little sped as the new seal gets broken in.  At any rate, it's delivering 24 ft-lbs now, so not bad power at all.


Shane

[Wolverineshooter]

My first generation Swarm Magnum is losing speed as well. Used to shoot CHP 14.3gr at 919fps when new. Now only 820fps. The breech seal still looks good so I suspect piston seal is worn. I shot less than 500 shots but is out of 5 years warranty period. This thread is very helpful

I am at seal level, looks elevation makes a big difference

[Thor760]

My first generation Swarm Magnum is losing speed as well. Used to shoot CHP 14.3gr at 919fps when new. Now only 820fps. The breech seal still looks good so I suspect piston seal is worn. I shot less than 500 shots but is out of 5 years warranty period. This thread is very helpful

I am at seal level, looks elevation makes a big difference

Greg.  I know you said your Breech seal looks good.  Have you tried the ol’ tissue test?  Place a tissue over the breech area and fire.  If the tissue blows off, moves a little or gets a hole torn in it from escaping air then it needs a new breech seal.

Also, what does your standard deviation look like over a 5 to 10 shot count?  Is the fps pretty consistent?  My Bone Collector started dropping fps as well as exhibiting poor standard deviation when the piston seal started going bad. It was all over the place with an avg 17fps standard deviation.

[Wolverineshooter]

My first generation Swarm Magnum is losing speed as well. Used to shoot CHP 14.3gr at 919fps when new. Now only 820fps. The breech seal still looks good so I suspect piston seal is worn. I shot less than 500 shots but is out of 5 years warranty period. This thread is very helpful

I am at seal level, looks elevation makes a big difference

Greg.  I know you said your Breech seal looks good.  Have you tried the ol’ tissue test?  Place a tissue over the breech area and fire.  If the tissue blows off, moves a little or gets a hole torn in it from escaping air then it needs a new breech seal.

Also, what does your standard deviation look like over a 5 to 10 shot count?  Is the fps pretty consistent?  My Bone Collector started dropping fps as well as exhibiting poor standard deviation when the piston seal started going bad. It was all over the place with an avg 17fps standard deviation.

I have nOt done the tissue test and will try that.

The speed is not consistent. Usually the first shot of the day can gets to 850fps, but then drops to 820fps and varies around that, plus minus 20fps.

Other suspect is the magazine, I will try a new magazine as well

[splitbeing]

Sounds like a seal issue, though could be other things.

[Wolverineshooter]

I changed to a new breech seal, shimmed the o-ring with dental work

[Thor760]

I changed to a new breech seal, shimmed the o-ring with dental work

What were the results following the breech seal change?  Any improvement?

[Wolverineshooter]

I changed to a new breech seal, shimmed the o-ring with dental work

What were the results following the breech seal change?  Any improvement?

I made 30 shots last evening. The speed was higher initially but went back to the same as before. I assume the higher speeds were from dieseling from moly I used on the metal around the seal. I concluded the seal is not the issue.

I have 3 spring guns non of them lost speed. All my piston guns lost speed. It is hard to imagine that the piston seal going bad on all my piston guns.

I wonder if the piston guns have a lot gunk and required a long time to get rid of dieseling. So the high speed and energy initially observed are just from dieseling and the speed drops once dieseling less. This may also explain why my first shot of the day is also faster but 5hen speed drops quickly. Somebody said that was due to heat generated from previous shots reduces air in the chamber with higher temperature. But I only observe this on my Gamo Swarm Magnum, not on my crosman piston guns.

Gamo offered to repair at a cost, including shipping likely to be $100. I am not sure if the gun truly has a problem, if the speed drop is due to loss of dieseling gradually. The worst would be they put in more gunk to have dieseling and higher speed due. If the speed is constant and the gun is accurate, that is better than have a high speed gun shooting erraticly.

[Thor760]

Sounds slightly lower than would be expected in a Magnum.  Perhaps others with that gun could chime in on their numbers.  It’s still making good power.  I would be interested in hearing the numbers on a 5-10 shot string.  If you can fashion a spring compressor, getting into these Gamos is not bad at all. 

Also how is the gun shooting?  Is it still accurate?

It is true newly bought break barrels need to ‘settle down’ their shot cycle and that sometimes takes a while as the parts break into each other and the stray factory lube diesels off.  Differs from gun to gun.  I know you got yours some years ago.

Personally I would advise against Moly anywhere around the transfer port or breech seal area.  I reserve it for the piston and the outer piston seal area.  Just my opinion.

AGD reviewed a gen 2 Swarm.
With 14.66 gr H&N FTT he averaged 940FPS.  Go to the Chronograph section of the vid.




He also did what looks to be an earlier generation and he averaged 911 FPS with 14.66 gr.

[splitbeing]

Did you check the piston seal? 

Also, what is the ES on your most consistent pellet over 10 shots?

[Wolverineshooter]

Here is the history of shots from my Gamo Swarm Magnum:

2018
Crosman premium hollow point 14.3gr (CPHP)
919fps (915 - 925fps for 5 shots), 26.8fb
JSB Exact 18.13: 800fps 25.8fb

08/25/24 to 9/12/24 in the order of time for each pellet

CPHP: (8/25) 869, 873, 845, 859, 848, 853, 846, 827, 827, 827, 827

H&N FTT 14.66gr: (8/25) 852, 818, 819, 824, 837, 807, 815, 821, 813, 829, 829, 847, 829, 829, 824, (9/11, shimmed breech seal) 856, 829, 822, 820, 818, 823, 816, 819, 820, 822

JSB 15.89gr (8/25) 815, 803, 802, 794, 793 , (9/4, new magazine) 821, 807, 795, 789, 784, 787, 782, (9/11, shimmed breech seal) 795, 790, 785, 785, 785

(1) my gun was never as fast as in the AGD review but not too shabby

(2) from 2018 to 2024, the speed drop is significant, energy dropped from 26fb to 22fb. My Crosman Mayhem has a similar drop (from 22fb to 18fb). In contrast, my springs guns have no drop in speed.

(3) the gun was very accurate back in 2018. It is still accurate but less so, likely because wider spread in speed

(4) I can rule out breech seal or magazine being the problem. Suspected piston seal but hard to imagine they will go bad on both my piston guns but not going bad on my spring guns. Crosman technician said piston had pressure even when not cocked and therefore piston seal more likely to fail than spring guns. I do not know enough to know if that is true.

If the gun can keep consistent speed I am ok with shooting it slower. But if there is a piston seal problem then I worry about it gradually degrade further.