AS568 Series O-rings

Our AS568 Series O-ring Size list

What is AS568?

AS568 stands for SAE AS568 (Aerospace Standard). It is published by the Society of Automotive Engineers (SAE) and titled "Aerospace Size Standard for O-Rings".

  • Status: Although it is named an "Aerospace Standard," it is actually the most universal O-ring size standard in the US and the global industrial sector.

  • Core Logic: It defines the Inside Diameter (ID) and the Cross Section (CS) of the O-ring, along with the corresponding tolerances.

The Five "Pillars" of AS568: Cross Section (CS)

The most distinct feature of AS568 is that it categorizes O-rings into 5 basic "Series" based on their Cross Section (CS) thickness. As long as you see the Dash Number, you immediately know how thick the ring is.

As a engineer, we need to memorize these 5 thicknesses (common metric conversions are in parentheses):

  • -0xx Series: CS 0.070" (approx. 1.78 mm) — The thinnest; used for precision instruments or tight spaces.

  • -1xx Series: CS 0.103" (approx. 2.62 mm) — Very common.

  • -2xx Series: CS 0.139" (approx. 3.53 mm) — Very common, the workhorse of industrial standards.

  • -3xx Series: CS 0.210" (approx. 5.33 mm) — Used for heavier-duty sealing.

  • -4xx Series: CS 0.275" (approx. 6.99 mm) — The thickest; used for large pipelines or heavy machinery.

While we often say 1.78mm or 2.62mm, when actually cutting molds, you must machine strictly to the imperial thousandths of an inch. Otherwise, tolerance accumulation will cause precision fits to fail.

Decoding the Dash Number 

AS568 uses a 3-digit number to denote specific sizes, in the format AS568-ABC.

  • The First Digit (A): Represents the Cross Section Series (the 0, 1, 2, 3, 4 series mentioned above).

  • The Last Two Digits (BC): Represents the sequential increase in Inside Diameter (ID).

Examples:

  • AS568-010: Belongs to the 0-Series (CS 1.78mm), ID sequence 10 (ID = 1/4" / 6.07mm).

  • AS568-110: Belongs to the 1-Series (CS 2.62mm), ID sequence 10 (ID = 3/8" / 9.19mm).

  • AS568-210: Belongs to the 2-Series (CS 3.53mm), ID sequence 10 (ID = 3/4" / 18.64mm).

Note: The ID growth in the last two digits is not strictly linear, especially in the smaller size ranges, so referring to the chart is necessary.

The Special -9xx Series

In addition to the 5 basic series, there is a special -900 Series.

  • Purpose: Specifically designed for SAE Straight Thread Tube Fitting Bosses.

  • Characteristics: Their ID and CS are designed to match specific tube thread sizes and do not follow the logic of the 5 basic series.

  • Application: Common in hydraulic systems and oil tube fittings. For example, AS568-904 corresponds to a 1/4" tube fitting.5. Relationship between Material and AS568

This is a common mistake made by beginners: AS568 is a dimensional standard, not a material standard.

When a customer says, "I need an AS568-214," they have only told you the shape and size. As an expert, you must ask:

  • "What material do you need? (NBR, FKM, EPDM, Silicone?)"

  • "What is the hardness? (Usually 70 Shore A, but could be 90 Shore A)"

  • "Are there color requirements?"

AS568 dimensions can be applied to any rubber material.


Few things to note about AS568:

  1. Tolerance Control:

    • AS568 has specific regulations regarding tolerances (usually referencing AS568 Class A).

  2. Gland Design:

    • AS568 defines not just the ring, but also the matching gland (groove) dimensions.

    • Compression Ratio: If you are assisting a customer with design, static seals are typically designed with 15%-25% compression; dynamic seals are lower, around 10%-15%. The AS568 cross-sections are designed to achieve these compression ratios with standard gland depths.