O-rings in every AS568 size and ISO 3601 metric

The first digit gives the cross section and the last two digits place the inside diameter within that series: 0xx is a 0.070 in section, 1xx is 0.103 in, 2xx is 0.139 in, 3xx is 0.210 in and 4xx is 0.275 in. Dash -001 to -003 are three small sections below the 0xx cord, and the 9xx series are boss gaskets for straight-thread tube ports. The number is a catalog lookup, not a formula, so use the size chart or the size page for the actual dimensions.

The tolerance grows with the size. A -214 is 0.984 in inside diameter plus or minus 0.010 in with a 0.139 in cross section plus or minus 0.004 in, while a -395 at 25.94 in inside diameter carries plus or minus 0.12 in on the diameter and plus or minus 0.005 in on the section. Each size page on this site lists both tolerances for that dash number.

Sometimes. The ISO 3601 metric list uses five cords, 1.8, 2.65, 3.55, 5.3 and 7.0 mm, that sit inside the AS568 cross-section tolerance bands, so the sections interchange; the inside diameters do not always line up. A -214 at 25.0 mm inside diameter matches the metric C0250 exactly, while a -006 at 2.9 mm is 0.1 mm off its nearest metric size and needs a gland check. Send both callouts and the groove dimensions and we quote the size that fits.

Those are O-ring part families tied to a material specification. MS28775 (now AS28775) is a nitrile ring to AMS-P-25732 for MIL-H-5606 hydraulic fluid; M83248/1 is a fluorocarbon ring to AMS7276 and M83248/2 a fluorocarbon ring to AMS7259; M25988 is a fluorosilicone ring to AMS-R-25988. The parts come from manufacturers qualified to those documents, and the manufacturer's certificate ships with every specification part.

The ERIKS O-ring Technical Handbook (chapter 12, read 2026-09-24) puts the static groove floor and top at 32 microinch Ra for liquids and 16 microinch Ra for vacuum and gases, with the groove sides at 63 microinch Ra. The Trelleborg O-Rings and Back-up Rings catalog (June 2024, read 2026-09-24) holds static sealing surfaces and grooves to 60 microinch Ra. For a dynamic seal Trelleborg holds the rod or bore to 16 microinch Ra, ground free of any spiral lead, and ERIKS asks for 8 to 16 RMS on the rubbing surface. The number alone is not the whole requirement: the lay of the finish should run with the ring, not across it.