CFX piston seal

[mthomas]

What recomendations/insights do you guys have on this. I am going to replace the seal again. I must have nicked something when reassembling a month or so ago. Velocity is at 715 fps w/jsbexact 8.4g. this seems low to me. maccarri has apex and tesla seals. What is your recomendation? Are there any other sources out there? WEhat is a source for the breach seals? Any help is appreciated.

[Gene_SC]

Let me ask you what did you clean the action with after you took it apart?

[mthomas]

after disssembly and deburring, i lightly crosshatched the compression chamber with some sandpaper, then cleaned with 90% isopropyl. then reassembled with tar on spring, very light, i used silicone grease on the rotary breach seal, sparingly, a bit of some moly behind the seal. One very loud report on the first firing, then it seemed good. recently noticed a slight decrease in apparent velocity. i will estimate 400 rounds since tune. borrowed a chrony and it was at around 715 to 720. I do not have baseline measurments after the tune, but I can here that the impact on a tree at 57 yards is a little later than it was immediatly post tune.

[Gene_SC]

If you removed the rotary breech and greased it then that is your problem. Did you use the old o'ring over? When tuning a CFX it is a shot in the dark most of the time. I suggest not removing the rotary breedh at all if if the fps numbers do not warrant it. I have pretty much stopped tuning the CFX because of the breech seal problems. If you notice the power plant is exactly the same as the S1K or Shadow, Hunters etc. You can achieve almost as advertized 1000fps. But in reality with the normal 7.9 pellet you will be in the low to mid 900 fps range with them. The way the CFX is designed it is not a positive air flow and hence you will never ever achieve 900 fps or above with a 7.9gn pellet. Best I have ever seen is about 900 fps and that was after seeing at least 100 of them go through the tuning process.

The breech o'ring does not seal 100% thus allowing air to escape around the breech o'ring.

[kiwi]

In the krals which are a copy of the CFX ..they leak quite bad...I put a O ring in with a larger section
go from 1mm to 1.5mm......Makes a big diffrents but can be a bugger to get in...And the breach is very
 hard to rotate for a while.....One other prob I have picked up is the front of the breach is not a flat
face... slightly high in the centre....and that stops the breach o ring "small one" getting full compression
against the barrel....Also check that the port lines up with the barrel corectly when in the closed position..
If the CFX has the same issues as the Krals...It will shoot way better with them sorted...

[mthomas]

when I tuned,I replaced the large breach oring with one the rich supplied. I feel that I was very gentle and easy at replacing the breach into the gun. As I stated, I think the velocity decreased after a while, but sooner than it should have. I will retune this gun and need advice on the seal you would select, and also on a source for new breach seals. Is there a difference between the apex and the tesla seals ? Is there another source for piston seals? Will the jm seals do what I need them to do? what kind of mgv sho"ld I expect. 900 sounds great, I see this as a /plinker, bhunter gun, not precision target. kiwi, where did you get the oversized oring?

[kiwi]

mthomas - 7/4/2009  5:43 PM

 kiwi, where did you get the oversized oring?

I just walk in the door of a engineering surply store...
In this case was a industrial berring and transmisson
out let...And asked for a 20X1.5mm O-ring..
and walked out with hand full @ about 25c each,,,,

I see all the time here and other forums where people
make a real drama out off getting   bits that can be
sorced from other places so easy.......

Bewilldering...........

[mthomas]

spent some time with a chrony yesterday. first a string with jsb exacts 8.4 grain avg mv was about 700. then shot a string w/ rws superdome 8.4 avg was about 800, back to the jsb, and it was still around 700. The jsb pellets have always been easier to load in the cfx, i guess they are significantly smaller and allowing a lot of blowby. the lower velocity may also account for the better grouping I get with them. The jsbs have a sticker on the back which says 4.52. As an experiment, I tried some predators, they also sho slow, around 730. Then I tried to swag the skirt on a jsb, to flare it a bit. this produced a slightly tighter fit at loading ,and an increase to about 740.

I am still going to re tune this gun, just to see the differance it makes, but it was interesting to see the variation caused simply by pellet diameter.

[mthomas]

by the way, these velocities are all at about 7400 ft above sea level. Is there a calculation for velocity effect versus altitude?