The Best O-ring Materal for PCP's

For 95% of uses in PCPs, Buna-N in 70 durometer is a great choice. For static O-rings in particular (O-ring stays in one place to hold pressure). Viton is also fine but it swells so if ever you need to reuse an O-ring, be prepared to wait several hours or a day before reassembly.

For dynamic O-rings—those that are subject to abrasion—polyurethane is a better choice. For the most part we are talking about bolt or breech O-rings. Granted, Buna-N will also last a long time in the vast majority of cases so don’t hesitate to go with it across the board. If the longevity of a bolt or breech O-ring actually proves to be a problem, it seldom can be laid at the feet of the material. More likely that the parts are rough and need to be smoothed, or a there is a burr or other sharp edge that needs to be removed, or a misalignment that is putting undue stress on the O-ring.

If a bottle that will be taken on and off, Buna or polyurethane 90. If it stays put, you can add Viton to the list.

NO VITON !!!! It swells under high pressure .... Use Buna/N in 70 or 90 duro pending application

Gotta say, this is some great info on o-ring applications. Thanks Nervoustrig and Motorhead.

its really not a guessing game every place an oring is used its specs have been established for a reason and they will vary, especially on hardness .... manufacturers DO have the exact specs on the orings including size material and duro .. getting the info out of them when they want to sell you a set of 5 cent orings for 100 bucks is another story ... all that said a 'universal' oring will do the trick usually but you generally would use black rubber to replace black rubber, and when it comes to more critical items like valves, regs etc yeah you want the exact part ..

View attachment materialchart.1637856179.pdf

https://www.allorings.com/o-ring-materials-comparison

It would have been nice if these could actually have displayed for me. I decided to use Viton based on these and other companies charts. Since they show lower permeability for Viton over Nitrile on both charts, I'm curious about the swelling issue. I've used a LOT of them and never noticed any swelling. Viton is stronger and more abrasion resistance with higher resistance to set, as well. Now most manufacturers use use Nitrile. It's likely because it's cheaper and more easily available in sizes needed but the lifespan of most airgun seals I've repaired was 7 to 9 years. The orings were hard plastic by then and had no sealing qualities at all. I'm fairly certain they were Nitile. Somewhere I read that 70 D Buna was good for a 1000 psi differential pressure and 90 D was good for 3000 psi. Clearances are important here. Higher strength or tear resistance would raise that. Polyurethane is the highest suitable for AGs but is typically used for it's strength and abrasion resistance like a bolt seal.

So anyway, I very much value Motorhead and nervoustrig's knowledge and experience and am always looking to learn. Just relating my own preferences and why.

Bob

My statement is that of PERSONAL EXPERIENCE and not what a chart says ... Call me pig headed or what ever.

Resealing nearly 100 PCP rifles every year and doing so for @ 7 years now have time and time again seen where VITON rings are used and if / when you can get the fitting there used on taken off ( yes they seal fine ) they remove ballooned and oversize.

Now if one is just doing "Service" no doubt a Viton ring can be used w/o issue, But if you are planning on tinkering with said PCP plan on replacing every ring if you take it back apart and wish to reassemble with an otherwise usable o-ring. * Now give said Viton o-ring a day or more to degas it generally will return to specification size and then fit correctly once again.



Now as to 90 duro rings ... because of them being substantially firmer having less willing compression yield, some instances such as internal air tube components they simply WILL NOT pass over a step or shoulder within tube and if forced simply nick or be damaged. Also STRETCHING a 90 duro over a male fitting ( Especially smaller diameters ) will just break them. 90 duro rings excel in high pressure application due to having least compression yield, yet that comes at a cost for ease of use in some situations.

POLY o-ring are the toughest wear wise in dynamic use but also the worst for shape stability taking a set far quicker than most commercial o-ring materials.



Yes indeed ... a "Per-Say" rubber o-ring material changes for a reason & gets further compounded used under HIGH PRESSURE.



Again .. only sharing first hand knowledge.



Scott S

Viton is stronger and more abrasion resistance...

Click to expand...

Just FYI, Viton's abrasion properties are actually poorer than Buna-N.



Not just on paper. A while back I tried Viton for the final stage O-ring of a hand pump. At the time, everything I had read was saying overheating was the main cause of failure so Viton's superior high-temperature rating (compared to polyurethane or Buna-N) looked like the way to go. It failed very early on. On disassembly, I noticed it had lost quite a lot of its cross section...classic abrasion failure.

Note also that Viton is rather susceptible to tearing. Where does that come into play? Well, regardless of the material, a high durometer is preferred for any application where abrasion is a factor. Stretching a small 90 durometer O-ring onto a pump rod (or a regulator piston or a check valve) is difficult to do and Viton is apt to tear. Heating it in hot water helps but a Viton is much more susceptible to tearing from what I've seen, in spite of the chart giving both it and Buna-N a fair rating.

Regarding durometer versus pressure, typically 70 durometer is suitable for working pressures up to 3000psi. Above that, 90 durometer is the way to go.

However it's all about the extrusion gap...how closely fitted the parts are. For the usual part tolerances, the guideline above is good. If the parts are a somewhat sloppy fit and a 70 durometer O-ring extrudes and fails, stepping up to a 90 durometer may take care of it. If you've ever noticed little bat wings on an O-ring, that's the material extruding into the gap between the parts.



A backer ring is another possible solution but I have never encountered an airgun where it was necessary.

On the NOTE of backer rings .... The WIDTH of the o-ring gland / groove needs to increase if a backer is to be used in most instances.



FYI