The ARCO 5379 Outboard Starter fits Mercury/Mariner, 40-50 HP, 10-tooth drive gear. It replaces the following part numbers: Mercury 50-73521, Mercury 50-73521T, Mercury 50-898265005, and Sierra 18-5605. Epoxy powder coated for maximum corrosion protection. Motor shafts induction case hardened at bearing surfaces for superior wear resistance. All armatures tested to withstand 50 times operation voltage (12V). All motors performance tested at 5 different load levels. All motors must meet or exceed OEM performance before being boxed.