Mercruiser Water Pump Repair Kit | Sierra 18-3321 Water pump kit includes base & housing for Mercruiser #1 stern drives with flush screw in front of gear housing. Sierra Mercruiser water pump kit with base For Mercruiser #1 Drives with flush screw in the front of the gear housing from s/n 2495186-2763441 Includes housing and base Replaces Mallory 9-48308