8-251 BLACK FKM 90D BACK-UP RING

APG stocks the VBU90251, an AS568-251 black Viton® (FKM) 90 Shore A Back-Up Ring measuring an inside diameter of 5.109 in (129.77 mm), a cross section of 0.139 in (3.53 mm), and an outside diameter of 5.387 in (136.83 mm). A back-up ring bridges the extrusion gap so the primary seal holds its shape under load.

Viton® (FKM) is valued for very low compression set at elevated temperature and outstanding high-temperature capability. It resists engine and transmission fluids, petroleum oils and fuels, and aggressive chemicals. Continuous operating temperature is published at -15°F (-26.1°C) to 400°F (204.4°C). Exposure to hot water and steam or low-molecular-weight organic acids will shorten service life.

Maintenance and OEM teams specify it across aerospace, high-temperature process equipment, and automotive fuel systems.

Order part number VBU90251, sold individually. Nominal inside diameter is listed as 5 1/8 inch. Confirm chemical compatibility, pressure and temperature against your own application requirements before installing.