Design Guidelines for o-rings

http://efunda.com/designstandards/oring/design_guidelines.cf...

In radial seals, the gland is defined by the Bore Diameter on the outside radius, the Groove Diameter on the inside radius, and the Groove Width in the axial direction (see schematic).

Inner Diameter
In order for the O-Ring to fit snugly in the groove, it is desirable to circumferentially stretch the O-Ring slightly. The recommended amount of stretch S is between 1% to 5%, with 2% as the preferred stretch value.
Operating Point O-Ring Stretch Value
% S
minimum 1% 0.01
maximum 5% 0.05
recommended 2% 0.02

The O-Ring inner diameter ID can be found from the recommended stretch Srec and the Groove Diameter Gd,
By stretching the O-Ring, we ensure that the O-Ring will stay in the groove and will not fall out or otherwise twist in some unpredictable manner during assembly.

Cross Section Diameter
The O-Ring is compressed in the radial direction when seated in the gland. Hence, one can think of the O-Ring cross-section as being pinched between the Bore Diameter Bd and the Groove Diameter Gd. In order for the O-Ring to be compressed when in the gland, its cross-section diameter CS must be greater than the total effective depth of the groove,