O-rings Information

O-rings InformationO-rings are solid-rubber seals that are shaped like a doughnut. When pressed between two mating surfaces, O-rings block the passage of liquids or gases.

Types of Seals

O-rings can form a static or dynamic seals. A static seal is where the O-ring does not move and is used simply for containing pressure or maintaining a vacuum. Dynamic seals can be reciprocating (like a piston and cylinder), or rotating (shaft rotating in a housing). Straight threads used with O-rings provide a better seal than tapered threads used alone.

A boss seal is also an O-ring, however it does not fit the standard sizes for an O-ring. A boss is a cylindrical projection on a casting or forging. The end of that projection is machined to provide a flat, smooth surface for sealing.

Application Methods

Axial squeeze and radial squeeze are two methods for applying an O-ring. An axial squeeze is when the ring is compressed parallel to a line drawn through the center or axis of the ring. In a radial squeeze the ring is compressed between the internal diameter (ID) and overall diameter (OD).


Imporant specifications for O-rings include size, material, hardness rating and features.


O-rings are typically available in standard sizes per industry standards. These include:

  • AS568 standard (001-475)
  • AS568 boss gasket (901- and up)
  • British Standard metric
  • JIS standard sizes
  • JASO standard
  • AS871
  • Custom sizes

O-rings InformationWhen custom o-rings are required, the width or thickness, insider diameter, and outside diameter are important to consider.


Common materials of construction for o-rings include:

  • Butyl (-75° to 250° F)
  • Cast polyurethane (-40° to 125° F)
  • Ethylene propylene (EPDM) (-70° to 275° F)
  • Fluorosilicone (-100° to 350° F)
  • Fluorocarbon/fluorosilicone blend (-25° to 400° F)
  • Highly-saturated nitrile (-40° to 300° F)
  • Hydrin (-60° to 300° F)
  • Neoprene (chloroprene) (-45° to 230° F)
  • Nitrile (Buna-N) (-30° to 275° F)
  • Polyacrylate (-25° to 350° F)
  • Polyurethane (Millable) (-40° to 180° F)
  • SBR (BUNA-S) (-30° to 275° F)
  • Silicone (-65° to 450° F)
  • Thiokol (-30° to 300° F)

Exact specifications should be clarified by the vendor, as chemical and physical properties of an O-ring can differ based on manufacturing technique.

Hardness Rating

The hardness rating on a Shore A scale for O-rings, can be:

  • <30 durometer
  • 30 durometer
  • 35 durometer
  • 40 durometer
  • 45 durometer
  • 50, durometer
  • 55 durometer
  • 60 durometer
  • 65 durometer
  • 70 durometer
  • 75 durometer
  • 80 durometer
  • 85 durometer
  • 90 durometer
  • >90 durometer


Common features for o-rings include:

  • FDA approved or 3A sanitary classification
  • AMS/MIL spec
  • Peroxide cured
  • Chlorinated


A-A-55801 - Rubber, silicone; packing, preformed (o-ring)

ASTM D1414 - Standard test methods for rubber o-rings

BS ISO 3601-1 - Fluid power systems - o-rings part 1

Image credits:

Applied Industrial Technologies | CT Gasket & Polymer Co., Inc.


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