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PostPosted: Sun Dec 02, 2018 8:14 pm 
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Joined: Thu May 04, 2017 6:41 pm
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Location: Vancouver BC
If anyone has experience with the GMAC FT kit I would like to know more about it.
I got a 1322 from the buy&sell forum and it seems to have the GMAC flat top piston kit installed.

I have two questions about the piston part.

Q: First what size o-rings does it use. Can I use the crosman factory rings?

Q: Why does the piston have a screw to adjust the length of the piston?

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PostPosted: Mon Dec 03, 2018 3:01 am 
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Location: Winnipeg, Canada
Not sure what makes the g-mac version any different from other flat top pistons available.
Probably not much. I don't know how to i.d. one as g-mac.

I think you will have to do a tear down to determine the o-ring size. It's not really that hard to do.
Unless there is a problem with the gun not pumping up, I'd leave it as is and put a drop of pell-gun oil on the piston o-ring. Don't for get to put a small drop on the pivot area while you have the oil at hand.

The threaded section is for adjusting the piston length for maximum efficiency. When adjusted properly, the tension of the piston face against the valve will be enough to hold the pump arm in the closed position. This will eliminate any dead air space between the valve and piston faces. I usually adjust mine to engage the face of the valve with the pump arm level with the mid point of the trigger guard front. This seems to work the best....

-D.S.

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PostPosted: Mon Dec 03, 2018 10:40 am 
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Location: Vancouver BC
Sorry, I meant to post a picture of the GMAC FT.
What I'm getting from your message is that proper adjustment means... as extended as possible and still be able to close.

This 1322 has a pivot point issue. The plastic barrel band was cracked and the roll pin too short to catch both holes.
The pin was canted inside the broken band.

So I threw it away and am going with a free float barrel. Seems to work well.
Issue was when I replaced the roll pin with drill rod the pump arm would not close because the pivot it is now straight and moved back a bit.
I gave the adjustable piston a few turns so it would close but now I think I need to fine tune that.

I want the piston face touching against the valve face.

Attachment:
0750-1.jpg
0750-1.jpg [ 40.49 KiB | Viewed 296 times ]

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PostPosted: Mon Dec 03, 2018 6:04 pm 
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Pushtone wrote:

I want the piston face touching against the valve face.

Attachment:
0750-1.jpg


Take the O ring off of your piston and adjust the piston length until the Piston face makes contact with the valve, while the cocking lever is still about 10-15 degrees from the closed position. Adjust the Piston length slowly to make sure they're is resistance when it meets the valve face but not so much resistance that you can no longer move the cocking lever to the "closed" position

This will ensure you transfer all of the air from the stroke into the valve and will leave the cocking lever locked into position from the "cam" type action of forcing the piston into the valve.

If this is not done properly you will not get all of the air transferred into the valve and your cocking lever will also be loose when in the "closed" position

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PostPosted: Mon Dec 10, 2018 3:20 pm 
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The gyst of the adjustment is on the last pump(s) the pressure is high which resists the plunger coming into contact with the valve and forcing all the air into the valve. The plunger and gun compress or stretch (depending on the part involved) a little under pressure, the more psi the more give it all has and the less valuable air get in. So if for example you adjusted the plunger so it's just kissing the valve, which seems obvious at first, then as you pump and psi rises the plunger will no longer touch the valve and it only gets worse as pumps increase. The fix is to adjust in some preload so it's kissing the valve at pump 10 and getting all the air in. The drawback of having it perfectly set for pump 10 is each previous pump, which is less psi, the adjustment is too much so you can feel the plunger binding. It's the worst on the first pump which you can really feel. No way round it unless all the parts involved were substantially stronger so they would not give under pressure, but they don't make said gun so this is how it is.
The big question is where do you set it so it's just right for the 10th pump? Most people just estimate, as you can see from other posts. Every mfg's plunger will have a different amt it compresses at 10 pumps, and likely so will your gun. If you wanted to get the max power out of the adjustment, but no more preload than needed, then I guess you'd have to adj it, crony several shots, adj a bit more etc until you see it no longer helps. Then back off to the least preload that still makes full power.
In addition to all this your gun may give a little more as it wears so to be perfect you may need to adj later. Catch 22; since this preload is in there it'll wear the gun faster since it's almost like every pump is the 10th pump. I say almost b/c it is only like that when the pump handle is closed, but since the new setup works better, each pump will be harder that oem. So overall it's more stressful on the pivot points so I'd be certain to lube them more often, and imo you should use grease instead, Moly paste would no doubt be better. I prep the parts by sanding, dry lube them, then use a similar grease mix I think is better than Moly. Who knows but it gives me a warm fuzzy.
Hope that makes sense, and helps :)

Fyi the Oring is an AS568-111


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