These o-rings make great breach seals!

[Tom Tucker]

I would like pass along a tip.

I was experimenting with different o-rings for the breech seals in my P1 and my P17s, and they all suffer from compression set to a degree.  I would install a new o-ring, and then after a few hours, a day, a week, a month, velocity would drop by 5-10 fps and the o-ring was smashed flat.  Sometimes this can compensated for with a little teflon tape or dental floss under the seal, but sometimes not.  I bought a whole bunch of o-rings from McMaster-Carr, and the very best ones for staying springy are these:

"Chemical-Resistant Super-Resilient Viton® Fluoroelastomer O-Rings"

https://www.mcmaster.com/viton-fluoroelastomer/=cd0480a267b145f0b548e6ea0c259595k36ky6v3

These were better than Buna-N, polyurethane, and standard Viton.

That's it!

[Stinger177]

Thanks for the tip Tom.

So which one of those 50,000 part numbers in the chart fits the P17 the best?

 

[Tom Tucker]

1288N109   

[Stinger177]

Thank you! 

[Dave S]

The only O-rings that work well with CO2 are 90 Durometer White Urethane. IMHO you're wasting your time with Buna-A or Viton. They are too porous for CO2. Been there...done that! dave

[Habanero69er]

+1 on the 90 Durometer White Urethane.

http://www.mac1airgunshop.com/urethane-mil-spec-90-durometer-orings-p/90urms.htm?CartID=1

[Stinger177]

The only O-rings that work well with CO2 are 90 Durometer White Urethane. IMHO you're wasting your time with Buna-A or Viton. They are too porous for CO2. Been there...done that! dave

That's good to know in the case of CO2 guns, but the P1 and P17 mentioned in the OP's first post are both over barrel cockers, not CO2. 

[Habanero69er]

Tim McMurray used 90 durometer urethane o-rings in my Steroided 'Dan & 342 because of their tough durability. They're not just for Co2 anymore.

[Stinger177]

Tim McMurray used 90 durometer urethane o-rings in my Steroided 'Dan & 342 because of their tough durability. They're not just for Co2 anymore.

Ah, my apologies. I understand now, and thank you for the tip. 

[Tom Tucker]

I use 90D polyurethane almost everywhere on almost everything.  They last for such a long time, and they get me a few extra fps in my pumpers.  But for the breech seals in my over-levers, these high-resilience Viton's are my new favorites.  The polys are good in that application too for the simple fact that they never seem to get torn or nicked, but they do get flattened permanently pretty fast.  Sometimes the chrony says it doesn't matter, sometimes it says it does...

[Dave S]

I didn't know the gun and assumed it was CO2. It's all good! For what you're doing...hey, whatever works! dave

[BillK]

I use these as the 70 Duro don't make the bolt stick as much.  CC is also very good.
https://www.theoringstore.com/store/index.php?main_page=index&cPath=367_1321

[sobdiver]

Good tip.  I may have to try some on my next trip to Mcmaster.  It looks like the equivalent at theoringstore dot com are .80 each.