There seems to be an amount of confusion surrounding o-ring names and types. This page has lots of information, especially useful in cases of buying your own seals locally or from the internet.
Most SAE (non-mectric, aka. decimal) o-rings are listed in a two-number form divided by a slash. For example: 12/70, 17/70, 12/90, 15/90, etc. This is refered to as the AS568A standard, or simply AS for short. It's the most common method of classifying o-rings in the States. The first number is the size of the o-ring, the second number is the durometer, both of which are explained later.
The first number in the name is the o-ring size. This number isn't the actual size by itself, rather it corresponds to a set of pre-defined sizes (below). All AS standard o-rings have a "height" of 1/16", meaning the o-ring will be 1/16" high if you lay it down on the table. In this situation, the term "width" refers to the diameter of the o-ring cross-section, which is also 1/16" since the ring is a circular o-shape (with a diameter of 1/16").
The diameter of the o-ring is listed as follows:
The fact that the width/height of each o-ring is 1/16" means the OD is always 1/8" larger than the ID (1/16" on each side, making 1/8" total).
I suppose I should also mention that there are such a thing as 2/70 and 1/70, however these o-rings aren't used very often (if at all) in paintball. Also, they're not standard 1/16" height.
If you have an unknown o-ring and need to know its size, you can measure it using calipers or just by placing it on a ruler.
The o-ring in this picture has an outside diameter of 1/2" inch, so it must be a size 12 (unless it's metric)
If an o-ring isn't measured in decimal and doesn't have the standard height of 1/16", chances are it's metric. Metric rings are measured in millimeters, generally by the inside diameter. Meaning, if you see an o-ring listed as 1x4-mm or 4x1-mm, chances are it has a height of 1-mm and an ID of 4-mm. However, if there is no "inside diameter" or "outside diameter" clarification after the size, you should verify the dimensions before purchasing your own. Personally I always label my metric sets with an "ID" tag to denote inside diameter, but others may miss this notation.
Large AS-standard Sizes:
Only o-rings with one or two digits are 1/16" height. O-ring sizes in the one-hundreds (110/70, 123/70, etc) have a larger height; there's also sizes in the two-hundreds, three-hundreds, etc. (it ranges from 100 to 999). These larger o-ring sizes aren't used much in paintball so I don't talk much about them on this page.
The second number in the AS term describes the thickness of the o-ring, which is officially known as the durometer. Durometer is the term used to descirbe the stiffness and density of just about any polymer material, from pencil erasers to chewing gum to the acetal in Tupperware. There are different classifications (called shores) of durometer but that's not important to this situation so I won't go into it.
Put simply, higher durometer o-rings are more stiff than smaller durometer ones. The so-called "default" o-ring durometer is 70, which allows for the best seal is most situations. However, there are some cases where a more stiff o-ring will be required, due to the design of the part using it, or the function of the o-ring. For instance, some clearance issues caused by a 70 durometer o-ring just barely not sealing can sometimes be cured by using a 90. Oftentimes the stiffer o-ring will help reduce leaks of this type becuase it doesn't adhere to the surface as loosley as the 70 duro ring does. However, in that sitution the 90 ring will tend to wear out more quickly.
Many manterials are used in this industry and others. Here's a short list of some of them and a few brief details thereof.
· Buna, Buna-T, NBR, Nitrile: These are different names for the same type of o-ring material (Butadiene Nitrile). These are the mose inexpensive o-rings available and are all black in color. Most metric o-rings are only available in Buna. Additionally, most specialty o-rings used in solenoids and other applications are also Buna. Standard AS Buna o-rings cost only a few cents each, whereas mectric ones cost around a dime each.
· Urethane: This is a more common material used in the industry because o-rings of this material are traditionally of higher quality than Buna, yet still remain relatively inexpensive. This means they tend to last longer. Urethane o-rings appear in various clear tints, depending on the manufacturer and method. Most of the time they will appear as a clear white color, but sometimes will carry a reddish or greenish tint (or any shade in between, brown, orange, etc etc). Further, 90 durometer urethane rings are often solid white instead of being clear colored. Available in many durometers. Urethane o-rings cost between 40-70 cents each.
· Viton: This is a greyish or blownish material that is often used in paintball regulators as 75 durometer o-rings. It is available in other durometers as well, but there's rarely a reason to use a Viton o-ring if not to use it in its 75 durometer condition (also, 75 duro rings aren't available in most other matierals, or at least the exact qualities will change). These cost between 10 and 20 cents each, depending on the size. 75 durometer o-rings help with the initial shock of being pressurized, which is why they're good in some regulator situations.
· HNBR, aka. Therban: The green o-rings that show up in SP and occasionally other guns are made from a material known as Therban. This material is chemically similar to Buna, except the materials are hydrogenized (addition of hydrogen). The resulting material tends to hold up longer than urethane or buna, while maintaining the same durometer as the lower-quality Buna rings. Although significantly more expensive, ideally this is the most long-lasting o-ring type available due to the long tolerances.
· Other matierals include silicon, neoprene, Polyurethane, Ethylene Propylene, Acrylic Polyacrylate, Perfluoroelastomer, and others. These aren't typically used in paintball though.
Despite the cost differences between o-rings, there really isn't such thing as a standard lifespan of a seal. It will vary depending on the frequency of use, surface finish of surrounding parts, weather conditions, lubrication, amount of particulates in the air source, etc etc etc. Additionally, not all o-rings are created equal. Due to this, nothing is truely "guaranteed" when it comes to the lifespan. A cheap Buna o-ring might outlast a Urethane one used in similar situations. You can never be sure.
Quad-rings are special o-rings made in an "x" cross-section shape to facilitate a double-sealing surface, manufacturered in Buna by Minnesota Rubber manufacturing.
The idea behind this is that the quad-ring wont be able to laterially twist and bend when the piston part moves. Quad-rings are designed to prevent this since they aren't circular, rather x-shaped and thus can't laterally twist (this is known as spiral failure). The other possible benefit is they are designed to allow for lighter seals since the surface of the quad-ring sealing against the part is smaller. This theoretically allows for less pressure required to actuate the part, but doesn't seem to have a practical benefit in paintball markers (this is because friction isn't the limiting factor in guns).
The only place you will want to install a quad-ring in a paintball marker is on the outside of a moving part (not the inside of one). This is how they're designed, and this is how they work best. Feel free to experiment with this on your own...but be prepaired for some rather obvious problems that may occur due to leaking issues. Quad-rings will work in some situations but not in others.
Quad-rings are occasionally available in kits but are used only sparangly in paintball markers. Special considerations have to be used when designing a part for use with quad-rings; some markers will leak severely if quad-rings are installed (two examples would be a Nerve or Intimidator, which both have some sizing issues and can't use them). Matrix DM3 bolts use a few for dynamic seals, as do 2004 model AKALMP markers, but other than that they're not very widely used due to the lack of practical benefit to markers.
Other customized o-ring shapes are also available should you require them in bulk. They require special molds per application so the upfront cost is high.
For reference, here are diagrams of o-rings used in all Smart Parts markers and accessories. And a Matrix bolt at the end.
Ion stock firing assembly
Ion firing assembly with Firebolt
Ion firing assembly with Nano bolt
Ion misc o-rings
Shocker SFT misc o-rings
Shocker SFT stock bolt, older versions
Shocker SFT stock bolt, current version
Shocker SFT Turbocharger bolt, older versions
Shocker SFT Turbocharger bolt, current version
Shocker SFT NoX NV bolt, v1.0
Shocker SFT NoX NV bolt, v1.5
Shocker SFT Evolve v1 bolt
Shocker SFT Evolve v2 bolt
Shocker SFT NDZ v2 bolt
Shocker SFT NDZ v3 bolt
Shocker SFT Freeflow bolts
Nerve firing assembly
Nerve LPR with extender components
Nerve misc o-rings
Impulse firing assembly
Impulse misc o-rings
Shocker Sport firing assembly
DM3 stock bolt
Evolve DM3 bolt
Matrix Fuse bolts - DM4, DM5, DMC, PM5, PM6, PM7
Matrix Fuse bolts - DM6, DM7
Matrix Fuse bolts - DM8, PM8
Ion vertical regulator
Max-Flo Preset system
Max-Flo manifold system
Max-Flo MF-I system
Impulse vertical Max-Flo
Shocker/Nerve vertical Max-Flo disassembled
Shocker/Nerve vertical Max-Flo assembled