Posted for "Gary Casey" <glcasey@adelphia.net>:

If I'm not mistaken the engine shown in the picture was an IO-520.  I have a
Continental drawing that shows the exhaust port being 2.36 inches
fore-and-aft and 2.00 down from the centerline of the cylinder and 12.77 out
from the centerline of the crank.  The spacing between bores is 7.31 and the
stagger between banks is 2.46.  My IO-550 drawing is incomplete, but shows
the port the same 2.36 inches from the cylinder centerline, but 2.56 inches
down.  Further, the IO-550 exhaust port is 1.69 diameter with a 4-bolt
flange while the IO-520 has a 1.547 diameter GASKET and a 2-bolt flange.  I
believe, but don't have proof, that the bore spacing and stagger of the 520
and 550 are the same.  They seem to be, like I say at work, exactly the same
except different.

Gary Casey