Question 1

How do I correctly measure an O-ring for replacement?

Accepted Answer

Measure the inside diameter (ID) and cross-sectional diameter (CS) using a digital caliper or dial thickness gauge. Take three readings of each dimension spaced 120° apart and average them. Calculate the outside diameter (OD) if needed using OD = ID + (2 × CS). Compare your measured values against the AS568 or ISO 3601 tables to identify the size.

Question 2

What is the difference between AS568 and ISO 3601 O-rings?

Accepted Answer

AS568 is an inch-based standard using dash numbers to identify sizes. ISO 3601-1 is a metric standard using actual dimensions (ID × CS) in millimeters. Many common sizes overlap closely, but ISO includes metric sizes not found in AS568.

Question 3

Can I use a BS 1806 O-ring in an AS568 gland?

Accepted Answer

Yes. BS 1806 sizes are dimensionally equivalent to AS568 sizes for standard dash numbers. A BS 1806-210 ring has the same dimensions as an AS568-210 ring.

Question 4

What measuring tool is most accurate for small O-rings?

Accepted Answer

For small O-rings (below 20 mm ID), an optical comparator or pin gauge set provides the most accurate ID measurement without stretching the elastomer. A dial thickness gauge with flat anvils is the best tool for measuring cross-section accurately.

Question 5

How do I convert an AS568 dash number to metric dimensions?

Accepted Answer

Look up the nominal ID and CS for the dash number in the AS568 table. Multiply each value by 25.4 to convert from inches to millimeters. For example, AS568-210 converts to 18.64 mm ID and 3.53 mm CS.

Question 6

What is an O-ring sizer?

Accepted Answer

An O-ring sizer is a measuring tool or reference chart used to determine the correct O-ring size for a given application. It can be a physical gauge set, a printed sizing chart, or a digital converter that matches measured dimensions to standard AS568, ISO 3601, or JIS B 2401 size designations.

Question 7

How do I use an O-ring sizing tool or gauge?

Accepted Answer

To use an O-ring sizing tool or gauge, place the O-ring over the gauge pins or cones to find the closest inside diameter (ID) match. For cross-section (CS), use a dial thickness gauge or caliper. A digital O-ring sizer can automatically convert measurements to standard dash numbers. Always verify both ID and CS, as the same ID can exist with different CS values in standard tables.

Dash	ID (inch)	CS (inch)
AS568-001	0.029	0.040
AS568-002	0.042	0.040
AS568-003	0.056	0.040
AS568-004	0.070	0.040
AS568-005	0.101	0.040
AS568-006	0.114	0.040
AS568-007	0.145	0.040
AS568-008	0.176	0.040
AS568-009	0.208	0.040
AS568-010	0.239	0.040
AS568-011	0.301	0.040
AS568-012	0.364	0.040
AS568-013	0.426	0.040
AS568-014	0.489	0.040
AS568-015	0.551	0.040
AS568-016	0.614	0.040
AS568-017	0.676	0.040
AS568-018	0.739	0.040
AS568-019	0.801	0.040
AS568-020	0.864	0.040

AS568 Series	Cross Section (inch)	ISO 3601 (mm)	JIS B 2401 (mm)
000	0.040	1.80	1.90
100	0.063	1.60	1.50
200	0.103	2.65	2.40
300	0.139	3.55	3.10
400	0.210	5.30	5.70

Dash	ID (inch)	CS (inch)
AS568-001	0.029	0.040
AS568-002	0.042	0.040
AS568-003	0.056	0.040
AS568-004	0.070	0.040
AS568-005	0.101	0.040
AS568-006	0.114	0.040
AS568-007	0.145	0.040
AS568-008	0.176	0.040
AS568-009	0.208	0.040
AS568-010	0.239	0.040
AS568-011	0.301	0.040
AS568-012	0.364	0.040
AS568-013	0.426	0.040
AS568-014	0.489	0.040
AS568-015	0.551	0.040
AS568-016	0.614	0.040
AS568-017	0.676	0.040
AS568-018	0.739	0.040
AS568-019	0.801	0.040
AS568-020	0.864	0.040

AS568 Series	Cross Section (inch)	ISO 3601 (mm)	JIS B 2401 (mm)
000	0.040	1.80	1.90
100	0.063	1.60	1.50
200	0.103	2.65	2.40
300	0.139	3.55	3.10
400	0.210	5.30	5.70

O-Ring Sizing Tool

Inch ↔ Metric

AS568 Lookup

Cross-Reference: AS568 / ISO 3601 / JIS B 2401

O-Ring Sizing Tool

Inch ↔ Metric

AS568 Lookup

Cross-Reference: AS568 / ISO 3601 / JIS B 2401