low alloy external backup ring 1.high accuracy 2.OEM offered ... plate St 52-3N round external backup ring with holes part namematerialdimension(mm)standardexternal backup ringlow alloy plate St52-3nD644*16 ... such as valves ,shafts ,flanges ,and their accesories.equipped with complete set processing machines:boring lathe ,miller ,punch press ... low alloy plate St52-3N round external backup ring with holes