Re: Ring pilot
One more time, it is exactly just what I said it is. It is to align the powerhead with the lower unit. It ensures the crankshaft is exactly centered ofer the drive shaft and lower unit bearings. It prevents drive shaft flexing and breakage due to mis-alignment. It has nothing to do with the crankcase halves.
As for the notch---take a look at it. Recognize anything? It is a modified magneto hold-down retainer ring. Somebody at the engineering department was clever enough to make use of an already existing part instead of investing $$$$ in a new casting. Once in awhile, they actually do use a bit of common sense up there.
EDIT: Sometime when you have the lower unit off, take a look at the water pump housing. It is machined to keep the gearcase centered under the exhaust housing----same reason as the pilot ring up above.
