8-390 BLACK BUNA 90D BACK-UP RING

APG stocks the HB90390, an AS568-390 black Buna-N (Nitrile) 90 Shore A Back-Up Ring built to an inside diameter of 20.953 in (532.21 mm), a cross section of 0.21 in (5.33 mm), and an outside diameter of 21.373 in (542.87 mm). A back-up ring bridges the extrusion gap so the primary seal holds its shape under load.

As a material, Buna-N (Nitrile) is widely chosen for strong abrasion and tear resistance, low compression set under sustained load. The material holds up against greases and lubricants, petroleum-based oils, and hydraulic fluids. The material carries a published service range of -40°F (-40°C) to 250°F (121.1°C).

You will find this material specified across automotive, agricultural equipment, and fluid power.

This item is sold individually under part number HB90390. Unit weight is approximately 17.569 g and nominal inside diameter is listed as 21 inch. As always, verify media compatibility and operating conditions against the requirements of your specific application.