Originally Posted by skylor
That’s interesting about the 390 C/D sump. Unless Lycoming used a different casting than the IO-360A sump, it doesn’t look like there’s room inside the sump to increase the bore 1” or so in diameter:
Same as the 360A. Ask Ron Moring about enlarging the flange. Apparently it can be increased enough to eliminate the adapter normally used to mount an FM-200, bore size 3-1/16". May not be able to fit an RSA-10.
It helps that the FM-200 bolt pattern is rectangular, 3.12 X 2.82. The RSA-10 bolt pattern is 3.375 square. Sketch below is to scale.
Beyond that, I dunno. Have not done it myself...but I'd like to.