Dimensions
Outside diameter is not a published AS568 dimension. The standard fixes the inside diameter and the cross section, and the outside diameter follows from them: OD = ID + 2 x CS. Every outside diameter on this site is that sum, not a figure read from a table, and it inherits both tolerances rather than carrying one of its own.
What the 3xx series is for
Heavy hydraulic and high pressure
Every dash number from 309 to 395 in this series has the same 0.210 inch (5.33 mm) cross section. Choosing a size is choosing a bore, and the cross section follows from the series - which is why a seal that is the right diameter but the wrong series will sit in the groove and still leak.
Fitting an AS568-377
The dimensions above are the free-state size of the moulded ring. Whether it seals depends on the gland: the squeeze applied to the cross section, the stretch on the inside diameter when it is fitted, and the room left for the material to expand into. Those come from the equipment maker or the seal manufacturer's gland tables, not from the size chart.
Material matters as much as size. The same AS568-377 is made in nitrile, fluorocarbon, EPDM, silicone and more, and they behave differently with temperature, pressure and the fluid being sealed. The dash number specifies the shape; it says nothing about the compound.
Frequently asked questions
What size is an AS568-377 O-ring?
Inside diameter 9.975 inches (253.37 mm), cross section 0.210 inches (5.33 mm), so an outside diameter of 10.395 inches (264.03 mm).
What is the cross section of a 377 O-ring?
0.210 inches (5.33 mm), plus or minus 0.005 inches. Every dash number in the 3xx series shares that cross section: heavy hydraulic and high pressure.
What is the outside diameter of AS568-377?
10.395 inches (264.03 mm). The standard tabulates inside diameter and cross section rather than outside diameter, so OD is ID plus two cross sections: 9.975 + 2 x 0.210 = 10.395 inches.
What is the tolerance on an AS568-377 O-ring?
Inside diameter is 9.975 plus or minus 0.055 inches. Cross section is 0.210 plus or minus 0.005 inches. These are dimensional tolerances on the moulded part and are separate from squeeze, stretch and gland design, which decide whether the seal actually works.
What this chart covers
This chart carries the five cross-section series - 0xx, 1xx, 2xx, 3xx and 4xx. It does NOT carry the 900 series, the 20 straight-thread boss gaskets AS568 also lists: those are boss seals rather than O-rings in the usual sense and are sized to the fitting, so they are left out rather than mixed in. Dash numbers -400 to -424 are carried and flagged: two of the four catalogues list them under AS568, and the AS568B list of 369 standard sizes does not.
Where these figures come from
Dimensions are as defined by SAE AS568B (aerospace size standard for O-rings). This site is an independent reference and is not affiliated with, endorsed by or approved by any standards body.
Every value on this page was cross-checked against 5 independent published sources: