PAI AN U-Cups are made to the dimensions of Military Drawing AN 6226.
Unit of Measure

Nominal I.D. (inches)

N/A 5/16

Nominal O.D. (inches)

N/A 13/16

Nominal C/S (inches)

N/A 1/4

I.D. (inches)

N/A .312

O.D. (Inches)

N/A .812