There are
five types of static o-ring applications: Flange seal, Radial seal,
Dovetail seal, Boss seal and Crush seal.
Flange Seal – In flange seal
glands, the two flanges are assembled with metal to metal contact.
So in fact there is no remarkable gap and no risk for extrusion of
the O-ring as long as the construction does not deform under system
pressure.
When system pressure is from the outside, the groove inside diameter
is of primary importance and the groove width then determines the
outside diameter. When system pressure is from the inside the reverse
is true.
Radial Seal – Because the metal parts are pressed or screwed together
there is always a clearance gap with risk for extrusion.
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Dovetail
seal – Also here there is a metal to metal contact as long as the construction
will not deform under system pressure. 
Boss seal – The groove dimensions are incorporated
in the standard dimensions.
Surface Finish Static
Grooves – Straight-sided grooves
are best to prevent extrusion or nibbling. Five degree sloping sides
are easier to machine and are suitable for lower pressures. Surface
finishes up to 64 to 125 RMS with no burrs, nicks, or scratches are
recommended. The method used to produce the finish is important.
If the finish is produced by machining the part on a lathe, or by
some other method that produces scratches and ridges that follow
the direction of the machinehead, a very rough surface will still
seal effectively. Other methods, however, such as end milling, will
produce scratches that cut across the o-ring. Even these may have
a rather high roughness value if the profile across them shows rounded
scratches that the rubber can readily flow into.
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