Mercury 35/40HP 2 Cylinder Outboards Service Manual PN 90-42794--1, Page 131Get this manual
Draining and Inspecting Gear Housing Lubricant AWARNING
If gear housing is installed on outboard, disconnect (and isolate) spark plug leads from spark plugs to avoid accidental starting when working near propeller1With gear housing in normal running position, place clean pan under housing and remove vent screw and gasket (a) and filldrain screw and gasket (b)
AWARNING
If gear housing is not removed before attempting to remove or install the propeller, remove (and isolate) spark plug leads from spark plugs to prevent outboard from starting accidentally3Bend tabs (a) away from propeller nut (b)