A propeller shaft has a tapered end that should exactly match the taper in the propeller bore. Any time you get a new shaft or propeller, you need the “lap” the two to get a tight fit between them. This fit is essential to minimize vibration and ensure the best transfer of power from the engine to the prop. Lapping is accomplished with lapping compound (Of course it is, right?). Lapping compound is just a finely regulated gritty substance that machinists use to make metal joints fit–like intake and exhaust valves in engines, or propellers on shafts. There’s no magic to it. Just slather the stuff on the taper of the shaft, and then rotate the prop around the shaft in a back and forth motion. Do this until you get a good, evenly dulled surface all around the shaft and inside the prop bore. If you’re unsure where the high spots are, you can paint a blue chemical dye called DYKEM on the shaft. Then scrub the prop back and forth and see where the dye transfers. Continue reading “Lapping the Prop to the Shaft”