"Bob Engelhardt" wrote in message news
I'm making a part that has an O-ring in a seat. I.e., not in a groove.
The part will be "loose" most of the time & I want the O-ring to
remain in its seat by having the seat diameter smaller than the O-ring's
nominal diameter. I.e., it'll be squeezed into the seat. How much
smaller can the seat be? The O-ring's nominal diameter is 0.7 & its
thickness is 0.1

Thanks,
Bob
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Orings are not compressible so the cross-sectional area of the groove while
sealed must be at least as large as the cross-sectional area of the oring.
Usually the groove depth is made 75-85% of the oring diameter to give 15-25%
crush for sealing. For really demanding designs Parker has tables of
different groove shapes for different oring materials used at different
pressures, but for normal not-really-critical stuff 80% works. Of course,
that wide groove lets the oring fall out when not sealed :-). The best
answer for that is a dovetail shaped groove with the top width a little less
than the oring diameter. The narrow top keeps the oring in place on
disassembly and the extra width at the bottom of the groove keeps the oring
from being extruded or cut.

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Regards,
Carl Ijames