Question for tuners

[poppacap1]

Will try to ask the question without adding too many boring details, Ive got an experimental gun "older shadow s1k" that I tuned recently, was probably as perfect a shooter as it could be , not leaving well enough alone I wanted to switch the apex seal out and try the tesla seal, with the Tesla seal installed i get eratic tight groups, dime size 3 shot groups on target then dime size 3 shot groups 2 inches high, then 2 inches low and so on.... So eliminating all other possible issues my question is has anyone ever had a similar accuracy issue just switching a type pr brand of seal that caused a accuracy issue, going to switch it back and see what happens this weekend but wanted some experienced input first... Thanks

[Motorhead]

If seals fit is such that it can chatter ( Think wind shield wipers on a semi dry windshield ) your velocity strings will be all over the place.
Sometimes a seal that fits a tad snug is better than a loose one. While a loose fit shoots faster, it lacks enough surface area contacting bore to be vibration dampening and can act weird.
Thus why most tuners in my experience tend to use larger seals sized to fit of just fit them a tad snug so that they need to break in.

Not saying this is your problem ?  just sharing my thoughts.

[CharlieDaTuna]

I don't know if I believe in the windshield vibration theory. Never ever heard anybody suggest that as an issue or even a possibility. Having tuned thousands of guns over many years I don't think that I have encountered anything like that and the fact that it is such a fast action how it could even be identified as such let alone possible. 

I would think that it is more likely a bit of excessive weepage of lube bypassing the seal causing changes in velocity but once the seal is broken in and especially if you crosshatched the compression chamber properly and used the correct lubes, it will smooth out and be more consistent.    Incorrect lubes and lack of or incorrect crosshatching is the biggest cause of velocity spread, especially in vertical deviation of POI. 

CDT

[Motorhead]

I knew it was a stretch stating that ... but feel a dry seal that squeaks does so because it is chattering, that is what makes the noise.
Even tho we may never hear an audible noise, of feel a shot cycle irregularity those parts in motion are vibrating and to the degree they slip or stick are somewhat dependent of pressure against the part and lubrication present.
Do agree speed is fast tho also being a mechanical devise the dynamics of parts sliding along having a smooth fluid motion has just these things happening, we just seldom or never think about it ?

In my background of High Speed 2 stroke engines, even a piston ring sprung out against the bore in which it travels at the certain surface speed will flutter and chatter along the surface.

That is the basis for saying what I did ... Tho it very well may just not apply ?

Sorry to of shared my deeper thoughts here ... I digress 

[thekid]

What exactly did you do in this tune of yours?

[poppacap1]

Interesting thoughts, remember I said that with the apex seal, it shot perfect, accurate , quiet and the shot cycle was soft, when I changed to the tesla seal the erratic accuracy issues started.
The original tune consisted of deburing, cross hatching the receiver, lightly polished the piston with 250, used jm moly and heavy tar as recommended and in correct amounts. The tesla is a tad more snug in the receiver than I would like, but wouldnt say overly so, still, maybe it needs to be broken in more, i guess since i sized the apex prior to instal and it was dead accurate from shot 1 i expected the same from the tesla seal right off the bat.

[thekid]

If you did not prep the receiver ( PTFE ) it can be erratic.
I changed seals in my stoeger ( when I had it) it was shooting smooth and consistent, but couldn't leave alone.
After the new seal was installed, velocity was low and erratic, removed everything Prepped the receiver, and cleaned it out with a rag, velocity returned and so did consistency.

Prepping:  cleaned out with denatured alcohol, then used ptfe grease to coat/burnish the receiver, cleaned it out with a clean rag, then gave the seal a little lick of silicone grease on the sealing lip, wiped it off then reassembled.
Obviously following the correct moly amounts.

[gene_sc]

Bob is right on the money about proper lubes and proper cross hatching. I might add and Bob knows this to. Not all air tubes are true nor are they all the same I/D.

This is a big reason why JM has changed seals over the years so much. JM and even Bob went through some rigorous testing a couple years prior to Bob's retirement on seal sizes. JM seals are good but he cannot manufacture seals of the proper size for every production springer made. Optimization is the best bet.

Now I have tuned a heck of allot of springers but not nearly as much as Bob or other pro tuners but I did learn a lesson early on. That was you just can't throw a new seal in a tune and think it will fit perfect ever time. Impossible. Guys like Bob and myself have a feel for the resistance a seal must have passing through the air chamber. To tight and you will end up with slow FPS. To loose and you will have seepage and excessive detonation. It has to be just right. The cross hatching stores the lube in its cross hatch. Seals slide over the lube in the cross hatch. A perfect fit and proper cross hatch will give you your best fps deviation.

But you also have to remember that not all chambers are true. Some will be egg shaped and there is nothing you can do to overcome this. Maybe a tight fitting seal but then you are defeating the purpose of the tune.
 
JMT

Gene

[poppacap1]

Well i would have to guess its the fit of this seal vs the apex that is the difference here, going to size the tesla and see what happens...

[tjk]

The easy thing to do first is to cycle a few hundred pelles through it and then do some group testing and velocity checks with a chrony. Never been a huge fan of the crosshatching technique,...just my opinion,... but a light buffing/burnishing of the compression tube with some moly certainly won't hurt anything. Wipe on and wipe off.  A slotted wooden dowell with a piece of cloth in a cordless drill makes this job a cinch. As for sizing, if you just continue to shoot the rifle as is, it will 'shape' or conform to the tube a bit more by itself. With a snug seal as you have mentionnedp, I don't think you will experience a whole lot of dieseling that can sometimes result in detonation with light weight pellets. From my personal experiences with seals and lubes, generally , snug seals are more forgiving of ''moderate" lubrication, whereas with less snug fitted seals, light lubrication works best. Again avoiding light weight pellets. Pellets (weight and fit) play a part in this due to how the seal reacts to the compressed column of air during the firing cycle. Seals types also add another twist in the equation. Yikes!!!!
I think it's cool you are trying your hand at self tuning, and with more practice, I have no doubts you will continue to improve your guns a little bit more at a time.

[nced]

Will try to ask the question without adding too many boring details, Ive got an experimental gun "older shadow s1k" that I tuned recently, was probably as perfect a shooter as it could be , not leaving well enough alone I wanted to switch the apex seal out and try the tesla seal, with the Tesla seal installed i get eratic tight groups, dime size 3 shot groups on target then dime size 3 shot groups 2 inches high, then 2 inches low and so on.... So eliminating all other possible issues my question is has anyone ever had a similar accuracy issue just switching a type pr brand of seal that caused a accuracy issue, going to switch it back and see what happens this weekend but wanted some experienced input first... Thanks

A bit of a disclaimer here..........not much "piston seal sizing" experience, and I have NO  experience with the shadow s1k.

Some aftermarket seals need to be "sized" after installation, so perhaps there is a "piston seal fit" issue between the two seals......one works well, the other not so well. Also, regular piston seals require quite a few shots to "stabilize" so perhaps the issue is one of inadequate break-in. The issue FOR ME with seals that required resizing was that it's pretty easy to "over do" the sizing, then you've trashed a $20 piston seal.

I did need to resize a Hornet seal a while back for my R9 and it worked out OK, however I've nixed the "size yourself seals" in favor of the Vortek VAC seal that didn't require any sizing in my guns, they just needed some time breaking-in.

If this info doesn't address the issue I apologize! LOL, there are reasons why I go to the hastle of installing oring sealed piston caps in my springers.......one of them is the fact that replacement seals when needed only cost a dime, and one of the other reasons is that they require little to no break-in! 

[poppacap1]

All of the input wether it pertains to my situation or not is valued, its part of the learning process, I guess with the apex seal i just got lucky with the right fit for that gun, everything else is the same down to the lube in the chamber.... Will see if the seal will break in shortly.