Scuba Diving O-Rings

This is good for buying your scuba diving O-Ring replacement online.

Sizing

In the USA the Society of Automotive Engineers (S.A.E.) publishes AS568B as an Aerospace Standard that specifies sizes for O-rings used in sealing applications. The equivalent British Standards Institution publication is BS1806, which lists all Imperial standard sizes. This standard includes all of the 5 main cross section groups in AS568, plus several sizes that are “between” AS568 sizes. In Australia BS1806 is widely referenced in many industries, including the dive industry.

The BS1806 number is a three-digit suffix that identifies the O-ring’s size. Technically, an O-ring size is listed as BS1806-016, BS1806-112, etc., but most people use BS and the three-digit number (e.g. BS112), or just the three-digit number (e.g. 112). The first digit denotes the O-ring cross section width: 0xx = 1/16-inch, 1xx = 3/32-inch, 2xx = 1/8-inch.

Please be noted that sizing an existing O-ring that has been in service can be error prone because O-rings can take a set that changes their dimensions enough to be misleading. Luckily, the sizes used in ‘user serviceable’ SCUBA applications are fairly regular, so it’s probably best to choose the size based upon the application, as below:

BS1806- OD ID CS SCUBA Application Seal
214 1-1/4″ 1″
1.000″
1/8″
0.139″
Cylinder with Large Neck/Valve (3/4″ NPS)
(All Aluminum, most Steel cylinders)
Duro: NBR 90, FPM 90
116 15/16″ 3/4″
0.750″
3/32″
0.103″
Cylinder with Small Neck/Valve (7/8″ UNF)
(“Genesis” 3500 psi Steel cylinders)
Duro: NBR 90, FPM 90
112 11/16″ 1/2″
0.500″
3/32″
0.103″
Standard DIN Regulator/Valve
(Nearly all DIN regulators)
Duro: NBR 70/90, FPM 75
111 5/8″ 7/16″ 3/32″
0.103″
Small DIN Regulator/Valve and Old Yoke Valves (You know if you need this one)
Duro: NBR 70, FPM 70
110 9/16″ 3/8″ 3/32″
0.103″
MDE Yoke Valves
Duro: NBR 90
015 11/16″ 9/16″ 1/16″
0.070″
Cylinder Valve Bonnet Nut
Duro: NBR 70, FPM 70
014 5/8″ 1/2″
0.500″
1/16″
0.070″
Standard Yoke Regulator/K-valve
Duro: NBR 90, FPM 70
013 9/16″ 7/16″ 1/16″
0.070″
Large Low-Pressure Port/Hose (1/2″ UNF)
Duro: NBR 70, FPM 70
012 1/2″ 3/8″ 1/16″
0.070″
High-Pressure Port/Hose (7/16″ UNF)
Manifold and Manifold Port Plugs
Duro: NBR 90, FPM 90
011 7/16″ 5/16″ 1/16″
0.070″
Standard Low-Pressure Port/Hose (3/8″ UNF)
Duro: NBR 70, FPM 70
010 3/8″ 1/4″
0.250″
1/16″
0.070″
Low-Pressure Hose/2nd Stage Regulator Cylinder Valve Stem
Duro: NBR 70, FPM 70
006 1/4″ 1/8″
0.125″
1/16″
0.070″
BC Power Inflator
Duro: NBR 75, FPM 75
003 3/16″ 1/16″ 1/16″
0.070″
High-Pressure Hose/SPG swivel
Duro: NBR 90, FPM 90

oring_sizes_bs1806.pdf and international_o-ring_size_standards.pdf may help to convert from BS1806 to other specs, such as AS568, BS1806.

Materials

O-rings can be made of a very long list of exotic materials and most are inappropriate for SCUBA applications. The materials commonly used in SCUBA applications appear in the following table.

Material Description
Acrylonitrile-Butadine Copolymers
(NBR or Nitrile)
Most O-rings used in SCUBA are made from Nitrile, a hydrocarbon based synthetic rubber. Nitrile offers excellent resistance to many oils and acids and has good physical properties. However, Nitrile is not a very oxygen resistant material and is not considered oxygen compatible. Nitrile is also often referred to as Buna-N.
Fluorocarbon Elastomers
(FPM/FKM/Viton®)
O-rings made of FPM/FKM/Viton® have been the preferred choice for oxygen and nitrox compatibility in SCUBA diving applications. FPM/FKM/Viton® is an elastomer with excellent oil and oxygen resistance at high and low temperatures, very good chemical resistance. Even for use with ordinary air, most experts agree that FPM/FKM/Viton® O-rings outperform common Nitrile O-rings.
Polyurethane
(PUR)
The milky white O-ring occasionally seen in the face of K-valves, sometimes called a 10,000 psi O-ring, is made of Polyurethane. Polyurethane is very abrasion-resistant, but it is sensitive to UV light (sunlight).
Ethylene Propylene Diene Monomer
(EPDM)
EPDM O-rings are becoming more common in SCUBA because some people feel it’s a safer material for use in breathing air systems. EPDM is a elastomer with excellent weatherability, heat resistance, dielectric qualities and odor-free characteristics. However, EPDM is less resilient and is not recommended for use with petroleum derivatives.

The Difference between FPM, FKM and Viton®
The terms FPM, FKM and Viton® very often cause confusion and lead to incorrect interpretations. All of these designations actually stand for one single base material: fluoro rubber.

  • FPM is the international abbreviation according to DIN/ISO.
  • FKM is the short form for the fluoroelastomer category according to the American standard ASTM.
  • Viton® is the registered trade mark of DuPont Performance Elastomers.
Posted in SCUBA

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