Here you can see the little centering wedges from the back side of Dan and Jim's crank extension.
I wasn't crazy about the design for the crank extension.
I admit it's very clever, but I don't like how many little parts are involved, and can imagine it eventually failing.
It works like a chuck that you center with centering screws.
Having made my prop hub I was aware that the front pilot and seal surface on my crank were running so true that you could
not see any runout.
The design also didn't match my method of bolting the starter flywheel on.