8-257 BLACK BUNA 90D BACK-UP RING

Part number HB90257 is an AS568-257 black Buna-N (Nitrile) 90 Shore A Back-Up Ring with an inside diameter of 5.859 in (148.82 mm), a cross section of 0.139 in (3.53 mm), and an outside diameter of 6.137 in (155.88 mm). A back-up ring bridges the extrusion gap so the primary seal holds its shape under load.

The compound resists water, many alcohols, and hydraulic fluids. Buna-N (Nitrile) is widely chosen for reliable performance in both static and dynamic service, low compression set under sustained load. Continuous operating temperature is published at -40°F (-40°C) to 250°F (121.1°C). Exposure to polar solvents or ketones will shorten service life.

Maintenance and OEM teams specify it across general industrial maintenance, construction equipment, and automotive.

Stocked as HB90257 and sold individually. Unit weight is approximately 2.1781 g and nominal inside diameter is listed as 5 7/8 inch. Confirm chemical compatibility, pressure and temperature against your own application requirements before installing. APG supports cross-referencing from drawings, samples and competitor numbers when a listed size is not an exact fit.