Flat top piston/valve ?

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fishinwrench

fishinwrench

Member
Nov 25, 2017
589
16
mo
I seem to be having trouble keeping the piston O-ring doing its job. I have had to replace it twice in 3 months. At first I thought I was over oiling so I cut back on lubing until I could actually feel it getting sticky.....but I just blew another one.

I'm beginning to think that the original cup seal was a far better system. Does anyone else have problems keeping the piston O-ring doing its job for a reasonable length of time ?
 
Sounds like extrusion failure so try a harder elastomer like Buna-N in 90 durometer.
For example https://www.amazon.com/gp/product/B0052AU0W6

Do you know the material and durometer of the O-rings that failed?
Click to expand...

I don't. They appear to be regular rubber O-rings. 

I got sick of the inconsistency and went back to the regular cup piston seal and cone nose valve, but I used the backend of the hotrod valve with it's enlarged exhaust port.

Very happy now. 528fps Hades with 16 pumps, and it's consistently accurate (1.5" groups) out to 45 yards, and 1" at 30 yards. Good enough for me.

I just don't feel like any O-ring is the right thing to put on a piston. A cup seal just has to be a smarter engineering decision.
 
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Well, I don't understand why you were soliciting information about the O-ring seal from the AGN community if you were just going to switch back to the cup seal anyway but I'm glad to hear it's working well again.

Indeed, the cup seal is preferable from a reliability standpoint. The advantage of the FT piston and valve arrangement is the ability to either build higher pressure or to get the same pressure as the pump cup with fewer strokes (because of less headspace / dead zone between the piston and valve face). No doubt those advantages come at a price of greater complexity, and reliability that ranges from somewhat poorer to vastly poorer depending on how it's used, how much cam-over it's adjusted for, what O-ring is used on the piston, and lubrication.

For what it's worth, the one pumper I still pick up from time to time is a hybrid of these two. It has a pump cup but it's set up to remove the dead space. It represents a good blend of performance and reliability in my opinion.
 
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I wasn't soliciting info on O-rings, I was asking if anyone else who had converted to flat top valve/piston was constantly blowing them out.....or if it was just me.

Seems to me that a smarter way to increase pressure (per same number of pumps) would be to REDUCE valve capacity instead of making it hold MORE air. I only need enough volume for 14" of barrel.
 
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My apologies, when I see the question “Does anyone else have problems keeping the piston O-ring doing its job for a reasonable length of time?”, I take it as seeking a solution to the problem of poor O-ring longevity. Reading it too literally suggests you’d like a show of hands from others who’ve had a problem with O-rings failing. The latter seems rather absent of utility. :)
 
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No it's more like this....

Everyone goes on about how awesome flat top valve and piston conversions are, so after shelling out the coin for valve/piston/and TP I expected to be more impressed. 

Power increase was so minimal and short lived that I guess I just wanted confirmation that everyone wasn't merely exaggerating.

Are you a psychologist by chance?
 
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Something was wrong if you weren't seeing substantial improvements not only in maximum achievable power but also in the power for a smaller number of pump strokes.

The gains over the stock setup are dramatic. At 10 pumps, it's an increase of 700psi->1100psi (57% increase). At 15 pumps, it's 900psi->1550psi (72% increase). The peak pressure advantage is 1250psi vs 2500psi which is a 100% increase.



Data courtesy of rsterne.

My results with a near zero cost modification boosted energy by 46% at 10 pumps.



These are not exaggerations.
 
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Well crap. I can't imagine what I could have done wrong. I'm an outboard tech and pretty well mechanically tuned.

Additional pumps after #12 did not get much harder, and the FPS gain per each 4 pumps up to 20 was so minimal that it made the effort not even worth it. I thought at first that I had over-lubed it, so I tore it down, replaced O-rings, cleaned and lightly re-lubed. That didn't help at all.....and after about 50 shots I blew another piston O-ring.

At that point I said screw it and went back to the original piston with cup seal, and swapped the flat face end of the modified valve out for the cone nosed end of the original valve. Also got rid of the BOSS TP and went back with the original TP that I enlarged a bit to correspond with the modified valve exhaust port and sealed it on both ends.

She shoots really good now at 525-530 with 15.89 Hades and 16 pumps. Could probably get 600 with 20 pumps but I don't wanna push it. I'd love to see 600fps with CPHP's but I don't see any reasonable way to get there and still keep it dependable. I plan to take this gun on canoe/camping trips and on lengthy bike rides......so I don't wanna be blowing O-rings or seals.

View attachment MTU1ODI0NTA1Ml85MzM1NDkyNTVjZTBlZWJjMDQ1Y2U2LjkzNjAxODgxX0lNR18yMDE5MDMzMF8yMTA2MjE0NjV+Mi5qcGc=
 

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