Engine alignment

I had an alignment problem with the starter shaft. If I rotated the clutch without the engine in place, I could feel the part of the shaft that fits into the clutch being pulled left and right. Play here is acceptable, but there should be a point where the stuff feels perfectly centered, where the stuff is not being pulled around.
Misalignment here will cause moderate but nasty high-frequency vibrations if the engine is mounted, and probably it will damage the small bearing in the clutch. I heard something being wrong there because there came a light rattling noise from my fan, as if it was running against a piece of paper or something... this was probably the small bearing which showed broken when I dismounted the whole stuff.
After long puzzling a friend came to the following. If you mount the coupling before mounting the engine (following the manual..), and you mount it perfectly tight, the clutch bell will be pulled slightly bend, because the shaft is thinner than the hole through the clutch and because the distance between the small bearing in the clutch and the bearing just under the hex cone becomes smaller as the clutch is bent more. See the figures.
From the figure you can also see that the vibrations can be expected to grow much worse once the cluch couples, as it will slide vertically through the clutch bell with each rotation of the engine. This may also be the explanation of the mysterious clutch failures reported by others.

Ideal alignment of clutch, starter axis and engine. Enlarge your browser window to enlarge the picture.

If the starter axis is fastened while the clutch bell is slightly rotated around its bearing, this situation is 'fixed'. see text.

The way to get the alignment right is to mount the engine and align it as possible within the play of the engine mounting bolts, before mounting the hex cone on the shaft. The only way I know to align engine with starter shaft is to feel whether the starter shaft can be rotated manually very smoothly. If there is a (possibly very slight) increase in friction the alignment is not OK.
To align the engine, I have to put thin o-rings on one side, between the engine and the frame. If you cant get the engine aligned, removing old loctite from the mounting holes can help giving just that little play needed.