New prop

It is possible (I don't know for sure) that some of the manuals say an 8-2 orientation. That would be as seen sitting in the cockpit. Usually we say 10-4 from the front of the aircraft.

10/4 makes hand prop starting logical. Not so easy
If 8/2.

My manual says 10/4, but it isn't clear if that is looking from the front, or from the cockpit. 10/4 from the front is the same as 8/2 from the cockpit. This conversation clarified all that for me. Thanks to everyone.

Thanks for your reply. I shared with my mechanic, I think the 10/4
Was what he was doing. The ercoupe Manuel was saying 8/2, but that was not happening.

If your prop is indexed incorrectly, even a well balanced prop will vibrate. Don't ask me how I know <grin>.

if this is the same prop that came off, it should go back on in the same place, it should be at 10/4 coming up on compression stroke for #1 cylinder, and 8/2 at TDC.
from the McCauley prop manual
Rotate the engine crankshaft so the number 1 cylinder is Top Dead Center (TDC) on its
compression stroke.
(6) Position the propeller on the engine crankshaft so the number 1 blade is clocked 60 degrees in
the direction of engine rotation from TDC.
NOTE: The number 1 blade will be located at approximately the 10 O'clock position when
viewed from in front of the engine looking aft.
NOTE: If you have not installed the propeller and spinner assembly on the engine before, the
use of a propeller sling and hoist is recommended to prevent accidental damage to
the propeller. If a hoist is not available, the propeller may be supported and lifted by
hand. Make sure an adequate number of people are available to support the propeller
if a hoist is not used.
(7) Install propeller on crankshaft flange.