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 Post subject: Sizing Piston Seals
PostPosted: Mon Jul 30, 2018 8:47 pm 
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Location: Thunder Bay,On.
What's the word on sizing air rifle piston seals for optimum fit?I always assumed seals came with the right specs for each gun and I'm wondering now if I'm missing out on extra fps and just plain smoothness of the shot cycle by not sizing them.Is there a specific diameter....or ft. pounds of force to be measured when installing? How easily should the piston move? .....Snug but not TOO snug is a relative term...

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 Post subject: Re: Sizing Piston Seals
PostPosted: Wed Aug 01, 2018 7:23 pm 
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Location: United States
As long as the tube and seal are lubed I can't see much point since the diff in resistance would be minimal, but I'd rather have the tighter fitting one as described further down: The groove is what concerns me far more, which is why I have several new seals that I will never use b/c the groove is unsat. The Trail XL has an completely unsat oem seal which imo needs to go first thing. There's two issues imo: One is the lost compression thanks to a large groove, which serves no purpose other than it's apparently easier for the mfg to make, the other is groove depth. A deeper groove means more of the seal is seeing pressure which forces more of it against the tube. The area of the seal toughing the tube, and how much force is pressing it against the tune is imo where you'll see power robbing friction. If for example the seal was long w/ a 1" deep groove then it just might stop the piston as soon as pressure built up, but would also no doubt be greatly affected by lube so it would also be inconsistent. All you need is enough sealing to not leak, which shouldn't take much of a groove. Another factor is how close the groove is to the edge b/c if closer that psi will more easily push the seal into the tube, or how hard/flexible the seal is. I have one that's more like hard plastic, another that's like a wet noodle. The soft one may work w/ no groove at all, the hard one would need a deeper and/or closer to the edge groove.
As you no doubt know I sand the face of my seals down to make them thinner which bumps compression and somewhat reduces the groove volume. This also smooths the shot cycle and bumps power. The only drawback is the groove is not squished down as much but the pros easily outweigh that con. So if you were to actually "fit" a seal to make a diff then imo it should be so fat that the groove is squished closed and your job is to sand until it's just open enough to work. Ideally it would come with no groove and you'd make your own to get the depth/width and position right. In the real world I don't think it's important enough to get that into it, just get a seal that has a reasonable groove, sand it's thickness down to ideal and call it good. You can play with the groove depth by filling the groove w/ silicone sealant and carving out however much, test, repeat. I started that test once but lost interest and didn't finish. All I did was fill the groove 100% just to see what it would do, and it proved that it certainly needs a groove b/c it lost power. It wasn't much, I think maybe 15-20%? That suggests to me maybe it barely needs much of a groove at all to seal it up. My guess would be a max of .010" wide (installed) and maybe .050 deep? Also note that under pressure the seal will sit flatter on the piston which can affect (close) the groove width.
So if they sell over size seals for the gun then I'd get the biggest one, sand the face, then check to see how the groove is when installed and only sand the OD if needed to open it. My guess is you won't need to touch the OD.
I normally sand the OD of the seal by hand w/ 320 but only to rough it up so I can impregnate it w/ dry lube. The nooks and crannies also hold oil so imo simply roughing it up will reduce friction far more than any sizing. Since it will stay lubed much longer there's also no risk of any spots going dry. A smooth dry seal pressed via hundreds of psi against smooth dry steel at high speed is the opposite of slick. I sand the tube for the same reason, just be sure you do it right, no brake hones etc.


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 Post subject: Re: Sizing Piston Seals
PostPosted: Wed Aug 01, 2018 9:56 pm 
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Joined: Thu Oct 04, 2012 10:27 am
Posts: 1108
Location: Thunder Bay,On.
Chevota wrote:
As long as the tube and seal are lubed I can't see much point since the diff in resistance would be minimal, but I'd rather have the tighter fitting one as described further down: The groove is what concerns me far more, which is why I have several new seals that I will never use b/c the groove is unsat. The Trail XL has an completely unsat oem seal which imo needs to go first thing. There's two issues imo: One is the lost compression thanks to a large groove, which serves no purpose other than it's apparently easier for the mfg to make, the other is groove depth. A deeper groove means more of the seal is seeing pressure which forces more of it against the tube. The area of the seal toughing the tube, and how much force is pressing it against the tune is imo where you'll see power robbing friction. If for example the seal was long w/ a 1" deep groove then it just might stop the piston as soon as pressure built up, but would also no doubt be greatly affected by lube so it would also be inconsistent. All you need is enough sealing to not leak, which shouldn't take much of a groove. Another factor is how close the groove is to the edge b/c if closer that psi will more easily push the seal into the tube, or how hard/flexible the seal is. I have one that's more like hard plastic, another that's like a wet noodle. The soft one may work w/ no groove at all, the hard one would need a deeper and/or closer to the edge groove.
As you no doubt know I sand the face of my seals down to make them thinner which bumps compression and somewhat reduces the groove volume. This also smooths the shot cycle and bumps power. The only drawback is the groove is not squished down as much but the pros easily outweigh that con. So if you were to actually "fit" a seal to make a diff then imo it should be so fat that the groove is squished closed and your job is to sand until it's just open enough to work. Ideally it would come with no groove and you'd make your own to get the depth/width and position right. In the real world I don't think it's important enough to get that into it, just get a seal that has a reasonable groove, sand it's thickness down to ideal and call it good. You can play with the groove depth by filling the groove w/ silicone sealant and carving out however much, test, repeat. I started that test once but lost interest and didn't finish. All I did was fill the groove 100% just to see what it would do, and it proved that it certainly needs a groove b/c it lost power. It wasn't much, I think maybe 15-20%? That suggests to me maybe it barely needs much of a groove at all to seal it up. My guess would be a max of .010" wide (installed) and maybe .050 deep? Also note that under pressure the seal will sit flatter on the piston which can affect (close) the groove width.
So if they sell over size seals for the gun then I'd get the biggest one, sand the face, then check to see how the groove is when installed and only sand the OD if needed to open it. My guess is you won't need to touch the OD.
I normally sand the OD of the seal by hand w/ 320 but only to rough it up so I can impregnate it w/ dry lube. The nooks and crannies also hold oil so imo simply roughing it up will reduce friction far more than any sizing. Since it will stay lubed much longer there's also no risk of any spots going dry. A smooth dry seal pressed via hundreds of psi against smooth dry steel at high speed is the opposite of slick. I sand the tube for the same reason, just be sure you do it right, no brake hones etc.



Thanks for the info on groove volume,etc. I never sanded seal faces before because I never thought it would make a significant difference in smoothing the shot cycle..so I will give it a go tomorrow when I once again take apart a few perfectly good shooting guns..lol... :D

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 Post subject: Re: Sizing Piston Seals
PostPosted: Wed Aug 01, 2018 10:14 pm 
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Posts: 1271
Location: United States
I consider it very important. Not sure if you've read about the details but you have to leave enough seal sticking out above the dovetail so it can't hit metal to metal. On a B18 I think ~.008" is about kissing, and that's measuring by pressing the seal down a bit so it's flat on the base as it would be under pressure. I press half of it down with a perfectly flat bar or whatever, then slip a feeler gauge in there. The remaining .008 is the seal being smashed on impact. If it kisses you can simply try again w/ a new seal, or sand the dove down a bit. It also removes any safety zone for a dry fire, which imo is why the seal is overly tall. I sand the dove smooth, like almost polished so any contact will be visible. Maybe even color it w/ a Sharpie?
Aside from the compression boost it nets you a wee bit more stroke too.
Let us know before/after power and smoothness.


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 Post subject: Re: Sizing Piston Seals
PostPosted: Fri Aug 03, 2018 9:48 am 
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Joined: Thu Oct 04, 2012 10:27 am
Posts: 1108
Location: Thunder Bay,On.
Chevota wrote:
I consider it very important. Not sure if you've read about the details but you have to leave enough seal sticking out above the dovetail so it can't hit metal to metal. On a B18 I think ~.008" is about kissing, and that's measuring by pressing the seal down a bit so it's flat on the base as it would be under pressure. I press half of it down with a perfectly flat bar or whatever, then slip a feeler gauge in there. The remaining .008 is the seal being smashed on impact. If it kisses you can simply try again w/ a new seal, or sand the dove down a bit. It also removes any safety zone for a dry fire, which imo is why the seal is overly tall. I sand the dove smooth, like almost polished so any contact will be visible. Maybe even color it w/ a Sharpie?
Aside from the compression boost it nets you a wee bit more stroke too.
Let us know before/after power and smoothness.

Finished with a somewhat radical result to seal sizing but I now have a 20fps increase with no other tuning changes.....just sanded the seal so it definitely adds power.....seal on left is original one from the gun after sanding...one on right is comparison....


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seals.JPG
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 Post subject: Re: Sizing Piston Seals
PostPosted: Sun Aug 05, 2018 10:58 pm 
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Location: United States
Sweet, glad to hear you tried it and posted results.
Here's a couple pix of ones i did, a 28mm Diana and a B18. You prolly seenem b4 but just in case.
Just fyi to anyone else; you can't sand willynilly! You have to be wary of the OD and thickness that works in your gun of you will damage the gun.


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 Post subject: Re: Sizing Piston Seals
PostPosted: Mon Aug 06, 2018 12:17 am 
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Joined: Thu Oct 04, 2012 10:27 am
Posts: 1108
Location: Thunder Bay,On.
Chevota wrote:
Sweet, glad to hear you tried it and posted results.
Here's a couple pix of ones i did, a 28mm Diana and a B18. You prolly seenem b4 but just in case.
Just fyi to anyone else; you can't sand willynilly! You have to be wary of the OD and thickness that works in your gun of you will damage the gun.


Yes....guilty of willy nilly...lol..I just eyeballed it and put the piston back in the tube to test the fit and also did the vacuum
test..(thumb over port to see if piston drops....)..luckily it worked out but in future....Ill measure as I go.......

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