Piston Seal question(s)

[ayerstg]

I tuned and put new piston seals (Apex) on my M 38 and M 24 and ran them across the chrony today.  I also dropped a GRT (not GRX) kit in my M 38, expecting to get an increase in energy.  What I am seeing right now is a decrease in BOTH rifles.  Details at German Gate thread

http://www.gatewaytoairguns.org/GTA/index.php/topic,11053.0.html

The pre-load on the GRT was much greater than the factory.  My top thought is that the seals need to be trimmed to fit better (they were really tight - a challenge to get in both guns without damage).  How likely is it that the tight seals are robbing me of 2 - 6 fpe?

And since my lathe is out of commission until after another payday and order to LMS, any suggestions on HOW to go about trimming them down?  I'm down to a drill press for machine tools until I get some parts ordered and installed...

Thanks in advance for any help!

Tom

[Pete]

hi..
you can do it with a drill press..
Put them on a good fitting bolt with washer and backing nut
spin them fast use a 400grit paper...

[JimL911]

New seals will need some sizing. Put the piston (with the seal on) in the drill and turn it on. Hold some sand paper against the seal as it spins. Check frequently, you can't put any back. I hold the tube vertical and let the weight of the piston just slowly drop while covering the transfer port with my finger. Remove my finger and it should drop freely. Cover and it should stop briefly. This should get you close to the "shooting" size.

[ayerstg]

Thanks, Pete and Jim!

...and... gracious the mess I am cleaning up from (apparently) too much tar!  I swear, I only put a little bit on 

Tom

[ayerstg]

Cleaned up the tar.  Left it off for now, as I suspect I'll be taking this apart a few more times. Note to self:  Next time just wave the jar around the general vicinity of the spring while muttering incantations.

Turned the seal some as suggested - in drill press while on piston.  Due to slightly loose front guide, I think it turned a little eccentrically.  It's still fairly snug, but... re-lubed (moly) and fired 5 rounds, then chrony 10 shots:

CPL 7.9 gr
793.2 fps
8
2.39
11.0 fpe

Getting a little spring noise (OK - it's a lot louder than it was with tar on the spring).  Less energy.  Wrong direction...   Just to be sure that the lighting hadn't changed and affected my readings, put one  lightweight thru it - nope; not only still sub sonic, but even less so (1009 fps)  Chrony reading fine (darn).

Next step will probably be to put original seal back in, although I may attempt to true up the Apex by taking it off and turning as Pete wrote.  Good thing I have another Apex spare + a Tesla.

Too dim outside to chrony now, so that part will have to wait

I really need to fix my lathe...

Umm - it just occurred to me that the new spring *may* be shorter than it was when I installed it.  Didn't measure it (should have) but I'll compare to factory when I take it back apart to get at the cussed seal.  What I remember is that when I first installed the new spring it was significantly proud of the back of the tube - an inch?  More?.  This time when I dis and re assembled, it was not sticking out as far.  Still a ton of preload, though (compared to factory).  <shrug>

Put the factory seal back on.  Chrony goes nuts under the tubes in the basement so have to wait for daylight.  Factory seal seems tight, too.  Oh, well.  And yes, the new spring is much shorter than it was before installation.  It is now essentially the same length as the factory spring.  Should it be?  I thought they came pre-set as part of a tune kit.  It's set now, that's for sure...  Anyone know if the GRT spring winds up at the same length as the factory?  Probably has a higher spring rate...?

[Mark 611]

you can probly still spcae up the spring a little, tho I'm not sure with out looking at what you have, if you take it back apart measure what you have, some times JM springs require some spacing to get the #'s back up.

[ayerstg]

Seems to want a larger seal.  Sizing the seal (I did get it concentric) dropped velocity to ~800 fps with 7.9 gr CPL.  Dropping an unsized seal back in brought it back up and I have a couple more to try as I get time.  More fiddling needed. 

The replacement spring is now same length as the original - ~10".  It is spaced out already because it has a top hat, a slightly thicker base on the rear guide, and a much thicker thrust washer on the rear guide (factory washer seems to be thick shim stock and worked fine).

I replaced the breech seal and also shimmed it out some with dental floss (it was handy) - not that I thought it really needed shimming, but...  it's ~ 830 fps with CPL 7.9 gr (didn't have time to shoot enough for good analysis). 

[Tuco]

Useful info on re-sizing piston seals - thanks Pete and Jim. Would you (or anyone else) clarify a point please? If you were to cur a cross-section through a seal, the leading edge is not perpendicular; it appears to be wedge-shaped. When trimming the seal should I maintain the same angle or is it OK to trim it perpendicular to the top of the seal? (Difficult to express, hope it makes sense.)
Chris

[ayerstg]

Lathe (and mill) back up and running, so I'm hunting for the "right" seal diameter better equipped.