Scuba Diving O-Rings

Many items of scuba diving equipment contain rubber O-rings that perish, dry out and develop cracks as time goes by. Even if the dive gear is not used, once it is a few years old, all the O-rings will need replacing.

An O-ring is a doughnut, or torus shaped seal typically used to prevent the passing of air or fluid. In other words, O-rings are used to keep fluid or air IN or OUT of a defined space. For example, an underwater camera housing uses O-rings to keep water out, and scuba regulators use O-rings to prevent precious air from escaping.

Scuba Diving O-rings

Just as barbed wire and duck tape seems to fix everything on land, O-rings and tie wraps seem to fix anything when you're at sea. These tiny rubber rings are crucial parts of a variety of scuba diving gear and equipment. O-rings prevent leaks at the seams of scuba gear and are a vital part of your life support equipment while diving. O-rings are used in scuba regulators, underwater lights and strobes, dive computers and cylinder/tank valves, and may often break due to pressure and wear and tear.

If you work on dive gear you know the importance of having the correct O-rings. Many of the O-rings used on cylinders, valves and regulator hoses are pretty much generic. Having a large selection of common O-rings is a must for the serious technical diver.

However, if you aren't well versed in the technical aspect of scuba gear, it's only advisable to attempt to change the O-ring on a scuba cylinder/tank valve (Yoke), or the face of your DIN regulator. Leave other O-rings for experienced professionals.

The information that follows should help you to know more about the O-rings used in various scuba diving applications.

O-Ring 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.

While it might seem convenient to provide a picture sizing chart, we've never seen an online version that could be reproduced exactly in a Web browser. However, you can download and print our Scuba O-Ring Sizes Chart (Adobe PDF | 356.37 KB) — Chart showing scuba diving O-ring sizes.

Please remember 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.

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

 
O-Ring 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.

O-Ring Hardness

O-rings are normally available with a hardness rating, as measured by an ASTM type-A durometer, of 70 (the "soft" kind) or more rarely a rating of 90 (the "stiff" kind). The soft O-rings are appropriate for dynamic applications where the O-ring is providing a seal with a constantly moving part, such as inside regulators. The stiff O-rings are more durable in static applications where the O-ring is providing a seal with stationary or occasionally moving parts, such as a valve.

O-Ring Colour:

O-rings are available in nearly any colour, although most commonly seen in black. Most FPM/FKM/Viton® O-rings are black. The FPM/FKM/Viton® O-rings typically sold for scuba diving are brown, simply to make them easily distinguishable from Nitrile O-rings which are usually black and EPDM O-rings which are often purple. Just to be clear, the COLOUR of the O-ring is MEANINGLESS.

Some people believe there is a difference in quality based on colour, but that is more likely attributed to unknowingly using soft dynamic O-rings in static applications. Our O-ring manufacturer states there is little difference in quality for O-rings based on colour alone, and independent testing supports that statement.

Some people also incorrectly believe a FPM/FKM/Viton® O-ring can be distinguished from a Nitrile O-ring by the surface appearance. Nitrile O-rings are said to have a shiny surface, and FPM/FKM/Viton® O-rings are said to have a matte surface. This is not a reliable indicator, and certainly not true if the O-ring has been lubricated.

How to Replace Your Cylinder/Tank Valve O-ring

Brass O-ring Tools Remove the old O-ring visible in the orifice of the tank's regulator valve. For this you should use a brass O-ring pick which looks similar to that of a dental pick (pictured right). This is because metal tweezers or other objects can damage the o-ring groove, and even the slightest dent of the groove can cause leaks. A plastic toothpick too is a safe tool to use if you don't have an O-ring pick.

Using the pick, scoop the old O-ring out and clean out the groove using the same plastic toothpick or professional pick. Look out for sand or debris and keep an eye out for any corrosion. If you spot any, you may want to have a professional take a look at it.

Check your replacement O-ring against the one you removed. Confirm they are of similar size and materials.

Push the new O-ring into the groove to replace the old one. It should slip in easily. If you have to stretch or force it, it's probably not the right size. Never use silicone grease. This is especially to be avoided if you are using a nitrox mix (a combination of nitrogen and oxygen). Silicone reacts with oxygen. Avoid using anything other than your finger or the back eraser end of a pencil to seat the O-ring.

Make sure the new o-ring you just put in doesn't have any nicks, cuts or stretch marks on it. If it does, replace it with a new one again.

Test the new O-ring, by attaching the first stage of your regulator like you would normally do. Slowly open the cylinder valve to pressurize the regulator. If you hear a loud pop or clap and air hissing out, the O-ring is not properly seated. Go back to the beginning and try again.

Apeks O-rings

This table lists the O-Rings used for servicing Apeks regulators for the following models:
* First stages: US4, DS4, UST, DST, FST(ATX100), FSR(ATX200)
* Second stages: T20, TX40/50/100/200, AT20, ATX40/50/100/200

Apeks ID BS Cross Sectn. Inner diam. SCUBA Application Seal
- 003 1/16 1/16 High-Pressure Hose/SPG swivel
AP6403 004 1/16 5/64 High-Pressure Hose/SPG swivel (less common)
AP1299 006 1/16 1/8 Apeks HP seat (1st stage)
AP1154 010 1/16 1/4 Low-Pressure Hose/2nd Stage Regulator
Cylinder Valve Stem
Apeks crown (2nd stage)
AP1409 011 1/16 5/16 Standard Low-Pressure Port/Hose (3/8 UNF)
Apeks adjusting screw (2nd stage)
Apeks conical filter (1st Stage)
Apeks FSR HP crown (1st stage)
Power Inflator quick disconnect oring
AP1445 012 1/16 3/8 High-Pressure Port/Hose (7/16 UNF)
Manifold and Manifold Port Plugs
AP1410 013 1/16 7/16 Large Low-Pressure Port/Hose (1/2 UNF)
Apeks balance plug inner (1st stage)
AP1159 014 1/16 1/2 Standard Yoke Regulator/K-valve
Apeks ATX adjuster screw (2nd stage)
AP1267 015 1/16 9/16 Apeks value spindle (2nd stage)
Cylinder Valve Bonnet Nut
AP1438 019 1/16 13/16 Apeks Venturi (2nd stage)
Apeks turret bolt (1st stage)
AP1420 024 1/16 1-1/8 Apeks Turret (1st stage)
AP1166 111 3/32 7/16 Apeks DIN Regulator/Valve
Apeks Yoke spare (2nd stage)
Note:Yoke valves often use BS112 or BS014
- 214 1/8 1 Cylinder with Large Neck/Valve
(3/4 NPS - All Aluminum, most Steel cylinders)
AP2041 2x1
(606)
1mm 2mm Apeks shuttle valve (2nd stage)
AP5711 2.5x1
(607)
1mm 2.5mm Apeks ATX adjuster screw nib (2nd stage)
AP1300 15x1.5 1.5mm 15mm Apeks HP balance plug outer (1st stage)
- 006 2.9mm 1.78mm Power Inflator spindle (Polyurethane)
- 012 1/16 3/8 Power Inflator cylinder inner oring (Buna)
- 112 3/32 1/2 Power Inflator cylinder outer oring (Buna)
- 204 1/8 3/8 Power Inflator oral inflate oring (Buna)

O-rings Available from The Scuba Doctor

At The Scuba Doctor Dive Shop we keep the following O-rings commonly used on the face of DIN regulators and cylinder/tank valves in stock for purchase in-store as emergency replacements:
N70-112 - Oring Black NBR 70 Duro BS112, 0.487 x 0.103"
N90-112 - Oring Black NBR 90 Duro BS112, 0.487 x 0.103"
V75-112 - Oring Brown FPM 75 Duro BS112, 0.487 x 0.103"
N90-110 - Oring Black NBR 90 Duro BS110, 0.362 x 0.103"
N90-014 - Oring Black NBR 90 Duro BS014, 0.489 x 0.070"
N70-111 - Oring Black NBR 70 Duro BS111, 0.424 x 0.103"

We also have a range of O-Rings and Lubricants available for purchase from our online dive shop.

WARNING:
Designers of modern SCUBA equipment often specify O-rings with special properties and materials for specific uses inside their equipment. The special o-rings are supplied as part of original equipment manufacturer (OEM) service parts kits. Substituting any other generic O-ring just because it's the right size can cause difficult-to-diagnose problems and degrade performance. Improper service could cause a failure which would have serious and life threatening consequences.

The Scuba Doctor Service and Repairs