MW 40-spline lightweight steel spool fits all 3.250" bore cases, including the 57440 MW aluminum and 57470 MW nodular units. Profile milling and cross-hole drilling saves 2-1/2 pounds over standard unit. Manufactured from through hardened 4140 high strength steel. Uses 7/16" bolts. 8.5 lbs. Installation - Service instructions can be found on Service Bulletin #0046.