Airgun o-rings?

[doninva]

Most O-rings for PCP air guns are Buna-N 70. Is there an advantage going to Viton, silicon, Teflon or Urethane. I am going to order online in bulk and would like to get the best material for the O-rings in my Marauder and Air Arms S 200 bolt probe and magazines. Thanks, Don

[nervoustrigger]

For the most part, no, there's no advantage to using premium materials...at least where static seals are concerned.

Some exceptions and other considerations would include:

1.  abrasion - If the O-ring is subject to repetitive sliding such as on a bolt probe, a material that better handles abrasion (like polyurethane) can improve longevity.  That's especially true if it slides past an edge that is somewhat sharp, though the best thing to do is dress the edge so it's no longer a problem. 
2.  preventing damage during installation- A high durometer (90A) polyurethane O-ring stands a better chance of surviving sliding past a slot or hole without getting nicked (compared to buna-N).  This is kinda like #1 except it may need to only slide past a sharp edge once, such as the O-rings used to install a tank block (e.g. QB79) or a gauge block (e.g. Benjamin Discovery).
3.  exposure to CO2 - Polyurethane is the best material when CO2 is involved.  For air, Buna-N works great.  Your context is PCP (air) but for some "dual-fuel" guns like the Benjamin Discovery, CO2 is a consideration.
4.  high temperature - High temperatures are usually of no relevance in PCP air rifles, but a peripherally related example involves the O-rings in high-pressure hand pumps.  For example, the -007 O-ring used on the inner piston (high pressure stage) of the Benjamin pump is notorious for being the weak link.  I've seen them fail sometimes due to heat but other times due to extrusion, meanwhile abrasion is a tertiary concern.  Therefore I have settled on 90 durometer hydrogenated Buna-N.

I hope this helps.

[grhouse]

Excellent answer. I recently bought a collect of o-rings in a kit of multiple sizes ans was wondering if any of them would be useful in my Discovery. Thanks for this post. Gene

[doninva]

Thanks Jason, that was great for someone o-ring challenged like me. Thanks again, Don

[TimmyMac1]

The 90 durometers Urethanes are the best for durability but they are very hard to install and they are harder to fit since they are hard. They must fit better to not be over-tight. The surfaces also need to be in better condition because a harder oring will not tolerate irregularities as well as soft.
Now the differences between Store bought 90's and Mac1 Mil spec 90's. The Mil specs will NOT absorb CO2 and Expand when containing CO2 Gas pressure. The store bought 90's will. They will eventually break down from CO2 saturation (within a half decade) where the Mil Specs 90's will time out in 25-30 years.
With the store bought 90's you will be good for years, the Mil Specs you are good for decades. I see it as worthwhile. I want parts that are compatible with the Labor going into the project.
I stretch the 90's slipped over tweezers spread with s straight blade driver. They are a Pain to install but the good news is the gun will probably never need another breech seal ever.

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

[Frank in Fairfield]

The 90 durometers Urethanes are the best for durability but they are very hard to install and they are harder to fit since they are hard. They must fit better to not be over-tight. The surfaces also need to be in better condition because a harder oring will not tolerate irregularities as well as soft.
Now the differences between Store bought 90's and Mac1 Mil spec 90's. The Mil specs will NOT absorb CO2 and Expand when containing CO2 Gas pressure. The store bought 90's will. They will eventually break down from CO2 saturation (within a half decade) where the Mil Specs 90's will time out in 25-30 years.
With the store bought 90's you will be good for years, the Mil Specs you are good for decades. I see it as worthwhile. I want parts that are compatible with the Labor going into the project.
I stretch the 90's slipped over tweezers spread with s straight blade driver. They are a Pain to install but the good news is the gun will probably never need another breech seal ever.

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

So, Timmy, if I buy one (some) of your o-rings will "I" be good for decades?
I (probably you too) am in my 7th...
Stay safe.

BTW, I removed my rear sight on my C9A and lost it.
If I get another from Crosman is there is trick to putting it back on?
Thanks.

[TimmyMac1]

The 90 durometers Urethanes are the best for durability .....They are a Pain to install but the good news is the gun will probably never need another breech seal ever.

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

So, Timmy, if I buy one (some) of your o-rings will "I" be good for decades?
I (probably you too) am in my 7th...
Stay safe.

BTW, I removed my rear sight on my C9A and lost it.
If I get another from Crosman is there is trick to putting it back on?
Thanks.

The breech seal is good for decades. Can't help you. just the gear. There is a trick to putting sights on. By stretching them they go on easier as well as come off easy and will not separate barrel from tube. I'd nix the wedges and glue the thing on with two part epoxy so the orientation of the rear and front sights can agree vertically. I have new sights for $5. $10 already stretched. $4 postage. The orings are $2 each. You ned as many as you have guns. If the buna is tight on the bolt go down a size so it stretches thinner.

TimmyMac1

[Frank in Fairfield]

I will make an order on Monday.
Concord HFT tomorrow.
I hope.
Easter and all.

Frank