Modding the Piston

[havenofear]

I just want some input on my o-ring piston design.  Kindly refer to the diagram.  I am using an oem Hatsan piston.  The attached piston seal is sanded slightly smaller than the piston diameter.  About 1.5mm away from the piston seal, I will cut a groove to accommodate an o-ring as a piston seal.  I keep the piston seal so that the length of the piston remains the same.  In this way, I have the best of both worlds.  If I want a parachute piston seal, I will take out the O-ring and replace it with a parachute seal and vice versa.  Do you think this will work?

[nced]

I just want some input on my o-ring piston design.  Kindly refer to the diagram.  I am using an oem Hatsan piston.  The attached piston seal is sanded slightly smaller than the piston diameter.  About 1.5mm away from the piston seal, I will cut a groove to accommodate an o-ring as a piston seal.  I keep the piston seal so that the length of the piston remains the same.  In this way, I have the best of both worlds.  If I want a parachute piston seal, I will take out the O-ring and replace it with a parachute seal and vice versa.  Do you think this will work?

A couple comments.
I'd be careful about machining an oring groove in the front of the piston shell, but then I don't know how the shell is attached to the piston "head".  If the shell is pressed or staked to the "head" then the oring groove would cut through the shell unless it's exceptionally thick. Another consideration is that you will have "double drag" when shooting due to the factory seal and the compressed oring seal. If the factory seal is reduced and simply used as a "bumper" then there will be a considerable amount of "lost swept volume" with the oring set so far back from the transfer port.

For my oring sealed pistons on HW and TX springers (I prefer orings) I make up an aluminum oring sealed piston cap that's fitted to the piston after removing the piston seal. That way the piston is un-altered and can be returned to "factory service" simply by drilling out the nylon retaining screws, or by unscrewing a retaining screw if the seal has it's that style of attachment.

HW77 oring sealed piston cap......
 

R9 oring sealed piston cap.......


TX200 oring sealed cap......


LOL....I even oring sealed my $19.95 Cummins Truckload Sale Chinese B3.....

[AudiS4]

I fail to see any point, of not using a parachute seal on a springer. I guess air gun manufacturers also fail to see the point of it... 

[SpiralGroove]

Hey Havenofear,

- What are you trying to accomplish by doing this Mod in the first place?  Is this an attempt to smooth the shot cycle?

- The piston head is swagged on or crimped on (see indents behind piston seal) ->nced is right.  I have had a loose piston head before and had to replace piston.  It probably wouldn't take much to break this joint by cutting the groove needed for the o-ring.

- nced - nice work on your pistons.  Why would one want to do this?

Good luck......

[Pappywith4]

I have heard heads being loose so I don't think there is enough metal there to do a seal groove !

[havenofear]

Thanks for your replies.  Actually, I want to experiment the design on a Crosman piston meant for the NPAW.  I have a spare piston.  When I looked at it, I find that the bottom of it is really solid with a lot of beef for me to cut a groove.  Why do I want to do that?  I have the Vortek O-ring piston seal that I fitted to my Gamo.  It is shooting fantastically and I gained a few fps with it from 735 fps (OEM seal) to 768 fps (with O-ring piston seal)using 10.3 gr pellet.  The Vortek O-ring piston seal is slightly larger for the Crosman NPAW.  I could not install it and totally damaged the seal when I forced it in to about 1 inch. 

I will cut a groove 1.5 mm from the base and fit a O-ring since I have the spare.  If it does not work out, I will just take out the o-ring and replace the parachute seal.  I also found out that the faster the piston compresses the air in front, the greater the power, not necessarily the swept volume of the combustion chamber (plays a part too).  So a stronger ram will compress the air faster than a weaker one producing higher mv and me.

[Pappywith4]

Let us know how it works out!

[nced]

I fail to see any point, of not using a parachute seal on a springer. I guess air gun manufacturers also fail to see the point of it... 

For ME the benefit was that my HW springers were less "temperature sensitive" with an oring seal than the old style HW parachute seal. I attributed this to the amount of "rubber and lube" riding the receiver wall and as the temps fluctuated during a shooting day the "rubber" durometer and lube viscosity would change which affected the motion of the piston and ultimately the trajectory of the pellet.

It was years ago at an early spring field target match in VA where both my brother and I did our zeroing during snow flurries, yet the temp rose to the mid 60s by the end of the match. During the match bot my brother and I had a 1" poi shift at only 20 yards and were missing more targets than normal. After that match I dispensed with "tar" on the spring using only molly paste which improved the resistance to temperature related poi shifting, but then (a couple decades ago) I hit on the idea of doing the sealing with only a 1/16" cross section oring to reduce the amount of "rubbing rubber".

I cut an oring sealed cap from aluminum, mounted it to the piston, then shot a few CPLs over the chrony. WHAT A SURPRISE , with less "rubbing friction" the CPL velocity jumped 50fps with the same spring compared to the factory parachute seal. This allowed me to reduce the amount of spring preload and still achieve a CPL velocity of 910fps which I preferred at that time. Since the functioning of the piston was more consistent with an oring seal I also got better accuracy and both my brother and I have been running oring piston seals in our HW springers ever since.

I did find that the latest design of the R9 piston seal with the reduced diameter base and "oring like parachute" gave similar velocity and performance to the oring sealed piston cap without the hassle of cutting and fitting a cap and actually used it for a while, but since have reverted back to the oring seal. Here is a pic comparing the various design changes of the R9/HW97 piston seal.......


Years ago there were other manufacturers that used oring to seal their pistons, however orings aren't very tolerant of sloppy receiver tolerances whereas the parachute seal can work with receivers of varying diameters and "ovality" along it's length so most manufacturers went the "cheap and dirty route" of parachute seals. Matter of fact, the high end high power gas rammed Theoben Eliminator utilized twin oring piston seals. Here is a pic of an Eliminator piston and you can see the double oring grooves near the face of the piston.......
 

Concerning the new style HW piston seal that does indeed approach the performance of my oring sealed piston caps, here is a thread from a UK airgun forum where the longevity of the new design parachute seal is questioned......and they are limited to 12fpe by LAW..........
http://www.airgunbbs.com/showthread.php?663309-new-Weihrauch-seal-test-and-some-thoughts

Anywhoo.....the point of "not using a parachute seal on a springer" is simply that orings work BETTER for me and my R9 IF there is a 30 degree temp shift during the day, I can achieve my preferred velocity with a less highly stressed spring, my accuracy is more consistent, a properly fitted oring will outlast a good aftermarket spring, and I can get a 50 count bag of military spec Viton orings for less than $10 so I roll in a new oring whenever I open up the gun for whatever reason.  LOL

[SpiralGroove]

Great piece of information nced,
Thank you