I have checked a number of engine/prop combinations with prop balancing
equipment and found the overall rock/yaw movement of any Continental or
Lycoming 4-cylinder sensitive to prop index.

There are 6 bolt holes in the crank flange and the prop will cover two. The
most perpendicular to TDC is the worst position. The best is the position
of a blade following TDC, the classic 10:00/4:00 suited for handpropping.
Even the intermediate position with a blade leading TDC will cause more yaw
on the aft of the engine.

My layman's mind attributes this to the firing order. The reaction and
rebound of a individual cylinder firing moving the crankcase.
I'm not sure but some of it maybe also be attributed to camshaft and
valve spring pressure. Valve train forces are much higher than you might
first suspect.

Most fixed pitch crankshafts do not have a particular index bushing that
constant-speed models have.
Yes, Lycomings have a index bushing for the ring gear position but its not
for a prop necessarily. One can install a fixed pitch prop in any of three
axis.

6 cyl Cont or Lyc cranks have a prop index bushing. Two choices, zero or
180 different. I know there has to be one but I have never seen a fixed
pitch prop on a 6-cylinder myself. I bet if the prop was indexed to the
middle two throws the engine would suffer from more yaw.

Offhand, I know of several instances when manufacturers recommend a change
in this reindexing of constant speed prop index. The first 300 serial
numbers of the Mooney M20J can be converted from the IO-360 A1B6 to A3B6 by
moving a prop bushing one hole counterclockwise which aligns it more closely
to TDC.
There is a Lycoming service bulletin advising about the same for some Aviat
Huskeys.
ANd there is a change thru the evolution of Grumman American service
manuals.

Oddest instance to me was hearing that some Europeans conducting tests on
aircraft with split exhaust systems have found the sound signature is less
when the prop is indexed a particular way, to be out-of-sync of the exhaust
note coming out the pipe.

Kent Felkins
Tulsa