Happy to move it to Tech Talk, Deejay - in fact if the thread continues, it needs to be there. I'll move it when I post this.
I agree that the popular engines in the time frame we are talking about were nominally made to standard dimensions. So, the engine output shaft alignment should always be the same, provided that the engines were accurately made, and that, on twin rail models, SB/Rover always installed the frame rails accurately then welded them in place without distortion. On single deck models, the proviso is that the decks and side plates were made accurately and consistently.
Aside from those possibilities of inaccurate positioning of the deck or rails, the other possible source of misalignment is that the mounting on the side plate for the self-aligning bearing could have either varied in position or been consistently wrong if the jig was made badly or designed badly.
So, if we are seeing inconsistency rather than consistent inaccuracy, that does not mean the problem wasn't misalignment - it just rules out consistent misalignment, such as might come from incorrect jigs for welding the rails or drilling the side plates. Inconsistent misalignment, which could be caused by sloppy operators in the factory, or bad maintenance of jigs, could still cause the crankshaft not to be pointed straight at the self-aligning bearing. However such misalignment happens, if we can establish that it sometimes does happen, and it tends to be present on mowers with cracked rails and/or loose clutch-halves, we might be a long way closer to clearing up some mysteries.
SB45 owners could check for misalignment (whatever its cause) fairly easily without needing advanced measuring equipment. I think all they need to do is remove the primary chain from the engine-shaft sprocket, disconnect the mounting of the self-aligning bearing so the shaft is supported only by the crankshaft via the clutch, and then see if the bearing's screw-holes remain in alignment with the side plate. To check for run-out as well, they can slowly rotate the engine's crankshaft (spark plug disconnected of course) and see if the bearing moves relative to the side plate.
