Re: Mercruiser coupler runout
Lots of thoughts: first, if your makeshift alignment bar is machined down 1/8 where it fits through the gimbal bearing, the bearing may not be set 100%. Next, I am not sure how you set the gimbal with any amount of force with just a long bolt tapped in. at any rate, I would first try to adjust the "set" on the gimbal bearing and see if I get any improvement.
Next, I would re-suspend the front of the motor, with rear mounts torqued, and be super careful about getting each front mount lower nut holding same weight, then turn carefully each nut the same from then on with any up/down adjustment.
Once I got the bar in/out as smooth as I could, I would test at 12:00, then 180, to 6:00 and try to even out just the up and down as best as possible. then start with 90 degree checks. as a last restort, you can loosen the front mounts to the stringers and try a tiny bit of leverage to get the front of engine to move sideways, even 1/16th might help. I have done this using a bar and a wedge shaped block of wood to hold the engine while I tested align bar again. then torque down, and final check alignment. my last thought would be a "runout" coupler. good luck!