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PostPosted: Thu Nov 25, 2021 1:59 pm 
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I'm wondering if anyone here has done a stuffed piston mod on a 13xx or 2289 pumper.
I'm having a hard time understanding exactly what this accomplishes, although I can sort of understand it make the pump stroke more efficient, but it seems to me that maybe 9 strokes with this mod would put as much air in the valve as 10 without, so it's not really a power mod IMO.
Ultimately, is it a worthwhile mod? Thanks.

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PostPosted: Thu Nov 25, 2021 2:33 pm 
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I haven't actually done the mod myself, but have received one 1377 with the mod. It will help pump efficiency, you would have to test the results on your own gun though, as the headspace on these guns I imagine vary.

If you want I could spin you up a delrin piston.


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PostPosted: Thu Nov 25, 2021 2:43 pm 
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I can imagine the only effect the mod does is to stiffen the piston. And, as you imply, the piston isn't particularly complicated. I may just turn one out of aluminum, since that's probably easier than doing the 8 pieces of pressed in steel. Thinking on it a bit more, what if I could find a steel or aluminum tube of the right ID, could I just "sleeve" the piston?

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PostPosted: Thu Nov 25, 2021 2:46 pm 
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I think that would work. I think the hardest part would be to get the plastic piston to spin true in your lathe. Then drill the pin hole and slot to match.


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PostPosted: Thu Nov 25, 2021 3:27 pm 
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This mod is done mostly by people who want the maximum fps by using maximum pumps which means 40+ pumps and with THAT kind of pumping comes great pressures pushing back on the piston which cause the piston to flex and allows a little less air into the valve with each stroke.So they stiffen it up with steel rods to prevent flexing and to get more efficient pumping but again its mostly for very high pumps...Stock setup works fine for the rest of us who are okay with 15 pumps or so... :D

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PostPosted: Thu Nov 25, 2021 3:36 pm 
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You can see benefits with lower pumps quite easily, especially if you play with reducing the valve volume.
It is not unheard of to get a 2289 doing 495 fps with 5 pumps, with air retention happening with 6+ pumps. I highly doubt you would get that result with a stock piston. Maaaaybe with the stock cup seal and a modified piston body.
I guess it all comes down to how much time you want to spend tuning it.


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PostPosted: Thu Nov 25, 2021 5:08 pm 
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I never did it myself, but a few years ago I bought a used 760 with the mod already done. I had to replace the piston because the steel pins were slowly ungluing off, and glue debris started to make their way in front of the piston. I replaced it with a bone stock one and never had any other issues.

The more modern pistons in 13xxP pistols are sturdier than the early ones, and a direct fit. Not the same plastic, much stiffer. Also make sure your valve hasn't been dimpled by the ''valve stop'' stamping in the tube, you'll lose power because of that. I saw that happen with the stock old-style piston. HTH.

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PostPosted: Thu Nov 25, 2021 5:34 pm 
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killercrow wrote:
You can see benefits with lower pumps quite easily, especially if you play with reducing the valve volume.
It is not unheard of to get a 2289 doing 495 fps with 5 pumps, with air retention happening with 6+ pumps. I highly doubt you would get that result with a stock piston. Maaaaybe with the stock cup seal and a modified piston body.
I guess it all comes down to how much time you want to spend tuning it.


good point...my mistake...there I go again with my "Amercianized" thinking. I assumed he was talking about maximum power with maximum valve volume which would require a good many pumps...yes of course you can build valve pressure quick by reducing volume... so one would want a sturdy piston :D

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PostPosted: Thu Nov 25, 2021 5:59 pm 
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MyCrosman wrote:
killercrow wrote:
You can see benefits with lower pumps quite easily, especially if you play with reducing the valve volume.
It is not unheard of to get a 2289 doing 495 fps with 5 pumps, with air retention happening with 6+ pumps. I highly doubt you would get that result with a stock piston. Maaaaybe with the stock cup seal and a modified piston body.
I guess it all comes down to how much time you want to spend tuning it.


good point...my mistake...there I go again with my "Amercianized" thinking. I assumed he was talking about maximum power with maximum valve volume which would require a good many pumps...yes of course you can build valve pressure quick by reducing volume... so one would want a sturdy piston :D


No you're also right on that account, makes it easier to pump into oblivion too :lol:


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PostPosted: Thu Nov 25, 2021 6:21 pm 
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OK, so I just got a new 1377, is it likely it has a stiffer piston than what was around when the stuffed piston mod was hatched?

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PostPosted: Thu Nov 25, 2021 6:56 pm 
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Just have a look at it.
It'll have some sort of glass reinforced plastic. You can usually tell by just looking. And the casting job will look a little better than the old ones.


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PostPosted: Fri Nov 26, 2021 5:00 pm 
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Dukemeister wrote:
OK, so I just got a new 1377, is it likely it has a stiffer piston than what was around when the stuffed piston mod was hatched?


Yes, if that's the 1377P with the redesigned grip and forearm, no need to do the mod in that case. If I compare my new 1377P to my older 1377 phase 3 (both un-modified), the new one definitely shoots harder (435 vs 410 fps).

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PostPosted: Fri Nov 26, 2021 7:13 pm 
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Yes it's the redesigned grip/forearm model, and it does look like glass fibre reinforced nylon for the piston. So I'll be skipping that mod. I was just looking through my bits snd oieces and found a nice hardened steel 3/16 dowel just the right lenth for the roll/spring pin replacement.


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PostPosted: Fri Nov 26, 2021 11:50 pm 
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PostPosted: Fri Nov 26, 2021 11:55 pm 
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old mod i did long time ago
stuffed piston old school way... :mrgreen:
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