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,
The difference between CS and the effective gland depth represents the compression C of the O-Ring (a dimensionless quantity),
C is required to be greater than zero in order for the O-Ring to be compressed. The recommended upper limit of C depends on the type of seal. In static seals, where the O-Ring is not in axial motion in the bore, the recommended maximum compression is approximately 40%. In dynamic seals, such as a piston moving inside a cylinder, the recommended maximum compression is somewhat less at 30%.