Choosing an OBR Mercury Verado inline-6 lower unit starts with the exact gearcase configuration, not horsepower alone. First, locate the complete engine serial number and any Mercury or OBR number on the original assembly. These identifiers help separate visually similar gearcases that may use a different shaft length, rotation or housing family.
Next, identify the required driveshaft length. The OBR variants on this page include 20-inch L, 25-inch XL and 30-inch XXL configurations. Then confirm rotation from the existing gearcase and propulsion setup. Standard and counter-rotation units are different products; the position of an engine in a multi-engine installation should not be used as the only confirmation.
Finally, match the original gear ratio and housing type. The listed 300 HP configurations are identified by OBR as 4.8-inch gearcases, but ratio mapping and some 200–275 HP technical details still require supplier verification. If any identifier is unavailable, submit the engine serial number and clear photos before ordering. Do not install the unit until the selected SKU has been confirmed.