There seems to be an amount of confusion surrounding o-ring sizing and selection. This page has lots of information, especially useful in cases of buying your own seals locally or from the internet.
Most non-mectric "SAE" o-rings are listed in a two-number form divided by a slash (example, 12/70, 17/70, 12/90, 15/90, etc.). This is considered the AS568A standard, or simply AS for short, which is the most common method of classifying o-rings in the United States. The first number is the size of the o-ring, the second number is the durometer or "hardness" (explained later).
The first number in the AS standard designation is the o-ring size. This number corresponds to a set of pre-defined sizes listed below. All AS standard o-ring sizes that have only two digits have a "height" of 1/16 inch, meaning the o-ring will be 1/16" high if you lay it down on the table. Other AS standard sizes are thicker but these are uncommon within the paintball industry. The o-rings being symmetrical means their heights and widths will be the same, as measured through a cross-section seen below.
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 rule (as shown below). This is easier with small o-rings since they tend to remain circular and not become oval-shaped, making them more difficult to measure by hand.
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 the SAE decimal system, 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, it's likelt to have 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; these size differences range between size 100-999. These larger o-ring sizes aren't used much in paintball so I don't talk much about them on this page. I'm sure you can look them up on other websites if you need the information.
The second number in the AS term describes the "hardness" 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 plastic 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 because it won't be as compliant with nearby components.
Many manterials are used in the paintball industry and others. Here's a short list of some of them and a few brief details.
· 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 and tend to be 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 a little more.
· Urethane: This material is more common in "moving seal" applications because it provides a theoretically better seal compared to cheaper Buna o-rings. This means they tend to last longer in some situations. 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). 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. Viton offers better compression so it's sometimes used as a bumper material, or used in regulator systems as a shock-absorbant pressure seal. These cost between 10 and 20 cents each, depending on the size.
· HNBR, aka. Therban: The green o-rings that show up in some equipment are made from this material. It's chemically similar to Buna, except the materials are hydrogenized (addition of hydrogen). The resulting rubber material tends to hold up longer than urethane or buna, while maintaining the same durometer as the cheaper-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 since they offer advantages in situations not often encountered in paintball equipment (chemical resistance, etc).
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, and others. Additionally, not all o-rings are created equal. Nothing is truely "guaranteed" when it comes to an o-ring lifespan; more expensive urethane o-rings are more likely to outlast buna seals, but it isn't a guarantee that you won't need to rebuild a pressure system using more expensive o-rings. Sometimes things just break and you end up rebuilding the parts regardless; you can never be sure ahead of time.
Theoretically these seals are less likely to laterially twist and bend using the moving surfaces slide against one-another. This is a characteristic known as spiral failure which can lead to shortened lifetime in other applications, such as hydraulics and other industrial components. In paintball, quad-rings have much less use, and in fact can cause certain components to leak if not designed correctly.
Examples of markers were quad-rings are appropriate include Matrix DM3 bolts or 2004 model AKALMP markers (Viking or Excalibur), but I don't know of any other cases where markers were specifically designed for use with quad-rings. They can sometimes be used in other markers as the moving seal, but often there isn't any practical benefit.
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
Invert/Empire Mini firing assembly
Empire Axe firing assembly
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
Custom Products vertical regulator