New versus old piston seal.

[Timmytree]

The Prosport hasn't had much use in the last 6 months so a bit of TLC was called for after I ran a quick string through the combro and realised power was down and a bit erratic. I knew the seal was due for renewal anyway, it had reached the point where the piston would drop almost under its own weight so not a lot of life left in it. With just a TbT guide kit and standard spring, no washers, it was putting AA Fields out at around 775 fps.
Fitting a new seal dropped the fps to 740 so I put in first 1 then 2 washers which got the figures back up to 770fps, I'll leave it at that because it's almost certain to rise after a few thousand shots. I decided to try an experiment so I stripped it again and put the old seal back on with the extra washers instantly giving me pellet speeds in the 800 - 820 region but not very consistent and it didn't feel good.
Now back to new seal and extra grease on the guide and it feels lovely, a quick session plinking confirmed the accuracy so I'm happy.
What I find interesting is the fact that the new seal with more preload feels better than the old seal with less preload. Maybe the piston needs an element of "braking" from the seal to help it stop at the end of the stroke? Maybe that tightness helps to regulate the way the spring acts when the sears are tripped?
Whatever the reasons, it feels better and is back to its usual form. But for those with springers, use a chrono if you change a seal, the change may be more than you bargain for.
Lastly, I decided to time myself doing a stripdown to remove/replace a washer, all the tools laid out on the table ready I managed to do it in a fraction under 5 minutes but I'm sure I could improve on that. May be the basis for a competition at the next meet?

[gary martin]

thanks for the post Tim. from your findings it looks as the better sealing properties of the new seal helps with consistancy of muzzle velocity, but needing extra spring preload to overcome the resistance created by the new seal, and the extra sealing properties create an air cushion when coming to the end of its stroke giving a sweeter firing cycle. just goes to show how vital a chrono is. thanks once again and keep us all posted with any other findings.
Gary.

[Timmytree]

That's just how I see it Gary, tight seal and preload keeping the power at a sensible level feels good, even better than short stroking or anything else I've tried.
The general opinion is to use less grease but I used more at the guide end and less at the piston end, I'll see if it migrates and ends up where I don't want it but at the moment I'm happy with the results. No twang, no buzz and very smooth firing cycle.

[Blackbaronfish]

Good to see your back into the swing. What grease did you end up using as I have a springer that needs fettling.

BBF

[Timmytree]

Bisley moly, that's all I ever use although any quality grease will do. What springer?

[Sake-san]

Welcome to 2015!, very thorough degreasing (because the old school and new don't seem to marry up so well!)) then just Finish Line Teflon grease. Sets up a bit waxy, does not migrate and hangs around for ever. As slippery as an eel, basted in KY jelly in a non-stick frying pan!. Does not seem to ignite at <12 FT/lbs but, as usual less is more...

[Blackbaronfish]

Bisley moly, that's all I ever use although any quality grease will do. What springer?

I dont know . I think its an Chinese import. It belongs to a neighbor that had it from a young age. Its got more battle scars than i dont know what .

BBF

[FPoole]

I've always used the JM small Apex seal. It's made of a softer material if it's the old blue colored one. They seem to last for ages and they do help soften the firing on FAC TX's.