Scuba O-rings

O-rings are small but essential parts of scuba diving gear, sealing connections in regulators, hoses, tank valves, and other equipment. A damaged or missing O-ring can cause leaks or equipment failure, which is why having the right replacements on hand is crucial for solving problems in the field.

This guide covers common O-ring sizes used in scuba diving, along with information on materials and durometer (hardness). Understanding these details will help you choose the correct O-ring for your gear and ensure a proper seal. I am focused on common field repairs, this is not an exhaustive guide and doesn’t cover every possible O-ring. While divemasters and boat crew are trained in swapping o0rings, technical divers, instructors and divers in remote areas need to be self sufficient in swapping o-rings correctly.

The information in this guide is for general reference and should not replace manufacturer recommendations or professional servicing. Using the wrong O-ring size or material can lead to equipment failure, potentially creating a hazardous situation. Always verify compatibility with your gear and, if unsure, consult a certified technician before replacing O-rings.

Understanding Durometer (Hardness)

Durometer is a measure of how hard or soft an O-ring is. In scuba diving, O-rings typically have a durometer rating between 70 and 90 on the Shore A scale.

  • 70 durometer: Softer, more flexible, and easier to compress, making them ideal for dynamic seals like moving parts in a regulator.

  • 90 durometer: Harder and more resistant to extrusion, commonly used in high-pressure applications like tank valve seals.

Choosing the right durometer ensures the O-ring can properly seal without being too soft (risking extrusion) or too hard (making installation difficult).

Common O-Ring Materials

O-rings come in different materials, each with specific properties suited for diving applications. The most common types include:

  • Nitrile (Buna-N): A widely used material that resists oil and fuel but has limited resistance to oxygen and high temperatures. Common in low-pressure applications.

  • Viton (FKM): More resistant to high temperatures, oxygen, and many chemicals, making it the preferred choice for high-pressure and enriched oxygen (Nitrox) use.

  • EPDM (Ethylene Propylene): Excellent for oxygen-rich environments, such as rebreathers and high-O2 blends, but should not be used with petroleum-based lubricants.

  • Polyurethane: Highly durable with excellent resistance to wear and extrusion, making it ideal for high-pressure applications like DIN regulator connectors.

common o-ring sizes

Proper Lubrication

Lubricating O-rings correctly can extend their lifespan and improve their ability to seal properly. However, using the wrong lubricant or applying too much can cause problems.

  • When to Lubricate: Most dynamic O-rings (those that move, like in regulator second stages) benefit from a light coating of lubricant to reduce wear. Static O-rings (those that stay in place, like tank valve seals) may not need lubrication if they fit properly.

  • Choosing the Right Lubricant: For oxygen-rich applications, use oxygen-compatible lubricants like Christo-Lube or Tribolube. Standard silicone grease should only be used for air applications, as it can react with high concentrations of oxygen and create a fire risk.

  • How to Apply: Use a small amount—just enough to create a thin, even layer. Too much lubricant can attract dirt and may cause O-rings to slip out of place, leading to leaks.

O-Ring Storage & Handling

O-rings can degrade over time if not stored properly. Here’s how to keep them in good condition:

  • Store in a Cool, Dry Place: Heat and direct sunlight can cause O-rings to dry out, crack, or lose elasticity.

  • Keep Them in Sealed Bags or Containers: Exposure to air and contaminants can cause deterioration. Some divers use small labeled ziplock bags or O-ring storage boxes.

  • Avoid Contact with Dirt and Oils: Touching O-rings with dirty or greasy hands can introduce contaminants that weaken the material. Always handle them with clean hands or gloves.

Identifying O-Ring Failure

A damaged O-ring won’t seal properly and can lead to gas leaks. Watch for these warning signs:

  • Cracks or Brittleness: O-rings that have dried out or are past their lifespan may crack or become stiff.

  • Flat Spots: O-rings can take on a flattened shape if compressed for too long, making them less effective.

  • Swelling or Deformation: Some materials can swell when exposed to incompatible lubricants or chemicals, preventing a proper fit.

  • Extrusion or Blowout: If an O-ring gets pushed out of its groove (often due to improper tightening), it may leak or completely fail.

If you notice any of these issues, replace the O-ring immediately. Carrying spare O-rings allows for quick field repairs.

Oxygen Compatibility & Cleaning

Oxygen-rich gas blends (such as Nitrox or trimix) require special care when selecting and maintaining O-rings:

  • Use Oxygen-Compatible O-Rings: Materials like Viton (FKM) are resistant to oxygen exposure, while standard O-rings can ignite in high-O2 environments.

  • Keep O-Rings and Components Clean: Any grease, dirt, or oil on an O-ring in an oxygen-rich system can increase the risk of fire or combustion. Regularly clean O-rings and fittings with an oxygen-compatible solvent.

  • Avoid Cross-Contamination: If you handle O-rings for high-O2 systems, make sure they don’t come into contact with petroleum-based lubricants or other contaminants.

O-Ring Kits & Emergency Replacements

Having a well-stocked O-ring kit can prevent minor issues from turning into dive-ending problems. Here’s what to include:

  • Common O-Ring Sizes: Carry spares for tank valves, hoses, regulators, and dive computers.

  • Essential Tools: A dental pick or O-ring removal tool makes it easier to extract old O-rings without damaging the groove. A small tube of appropriate lubricant is also useful.

By keeping an O-ring kit in your dive bag, you can quickly fix minor leaks and avoid canceling a dive over a simple issue.

Finger-Tightened Connections

Many technical divers choose to finger-tighten certain connections, such as 2nd stage regulators or high-pressure hose fittings, to make it easier to swap components. While this might seem convenient, it can lead to serious problems, particularly O-ring extrusion and failure.

O-ring extrusion occurs when high pressure forces the O-ring out of its groove, causing leaks or a complete seal failure. This is more likely to happen if the connection isn’t tightened properly, allowing the O-ring to move under pressure. A loose fitting can cause the O-ring to blow out suddenly, leading to an immediate gas leak or loss of gas supply. Often the excursion can be fixed by loosening and then tightening the connection however extrusions, especially on 2nd stage regulator hoses, can lead to torn O-rings, requiring a replacement.

extruded o-ring

Extruded tank valve o-ring

To prevent O-ring extrusion and ensure a proper seal:

  • Tighten connections properly: Always use the appropriate tool (if required) or tighten by hand to manufacturer-recommended specifications. Avoid over-tightening, which can damage the O-ring or the fitting itself.

  • Inspect O-rings regularly: Check for signs of wear, flat spots, or extrusion.

  • Use the correct durometer O-ring.

While it might take an extra moment to secure a fitting properly, it significantly reduces the risk of O-ring failure, ensuring a safer and more reliable dive.