0.313 inches nominal outside diameter, 0.139 inches minimum and 0.145 inches maximum aperture diameter, one-piece construction, glass and rubber synthetic material, mil-p-997 mil spec single material response material document and classification, circular style designator

A thin flat metal or plastic item of various peripheral shapes having a solid or laminated cross-section. It is designed to maintain a predetermined distance between two surfaces. For round items without a hole and having a solid rectangular cross-section use disk, solid, plain. Items with either (1) a solid (not laminated) cross-section, or (2) a non-separable laminated cross-section with one hole centrally located, must have a thickness less than 0.006 inch (0.152 mm) for items less than 1/4 inch (6.35 mm) outside diameter or shortest distance across peripheral flats, and less than 0.016 inch (0.406 mm) for items 1/4 inch (6.35 mm) or more outside diameter, or shortest distance across peripheral flats. For items with thickness greater than those indicated above, see spacer, plate; washer, flat and spacer, ring. When thickness and/or outside diameter or shortest distance across peripheral flats is designated as a tolerance dimension, the maximum dimension will be used to determine compliance. See also shim stock, laminated; shim stock assortment, gasket; and bearing, washer, thrust.