We are moving the flywheel on the crank pin. Spreading moves the pin in the wheel sideways. Out of round, the wheel radically on the pin.
Using the index finger on both hands, stuck straight up, next to each other,
Fingers closer at the tips, from the hands, if we "wedge" them open farther at their tips, to past equal, then let them return, we straighten the wedge.
If we have the fingers straight and equal, but one tip offset from the other, the wheels are not true, and we have to move them on the pins to straight.
Not all pins insert into flywheels dead straight, some can insert a touch canted offset, to fix, we align. No jig will be totally straight align for every pin press in.
Yes, I know, rudimentary, but effective.
Crankshaft alignment is extremely important to rods tracking straight, quite different than the "fit running side clearances" of a ball bearing running inline with other ball bearings.
Case in point, the old Honda single crank 800cc V5 was so "touchy" on crank set in the cases, race mechanics in now, MotoGP, were forbidden to split engine cases, in fear of actually binding it in the cases when reassembled. Honda had ONE tech, at the race department in Japan, that set all the cranks in the cases. NOBODY else was allowed to do so. If there was a question/problem with the crank, the crank/case set was replaced as a preassembled unit only. It was well known that if a crank wasn't set correctly in the cases, up to 20 horsepower could be lost from ball bearing side binding.
All that was with the same C3 (EMG) bearing looseness we have on our cranks, so, straight, true, aligned and set in place is essential for proper engine operation.
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