low alloy external backup ring 1.high accuracy 2.OEM offered 3.longer service life 4.quick delivery and ... low price always low alloy plate St 52-3N round external backup ring with holes part namematerialdimension(mm)standardexternal backup ringlow alloy ... processing.such as valves ,shafts ,flanges ,and their accesories.equipped with complete set processing machines:boring lathe ,miller ,punch press , ...