A 5/16-inch measurement equals 7.9375 mm. Confirm the unit and whether this dimension describes the passage, mounting hole or another feature.
The nominal size does not specify hardness or overall thickness. Check those values on the drawing, together with groove dimensions, flange clearance and tolerances. A part with a 5/16-inch internal passage may require a substantially different mounting opening.
Select the compound using the actual temperature and exposure requirements. Compare grade-specific data rather than assuming a rubber family always withstands or fails at an unspecified high or low temperature. Verify fit before production use.
