ORIGINAL: Artisan
Xjet, as an engineer you should know that there is not enough latitude in the production of these engines for some to be significantly more out of balance than others.
I'll bet those complaining of shaking are those trying to break-in their engines while flying. This is not meant as a shot. But I'll bet it is the truth.
If we were talking GMS or some other cheap Chinese engine I'd agree -- but the Saitos seem to be very well made and decidedly well made. When you consider the magnitude of the vibrations experienced by some, the kind of mis-machining or deviation from spec would be obvious to anyone with a dial-indicator or set of calipers and micrometers.
Both my "smooth" and my "jackhammer" Saitos show exceedingly good fits and tolerances. There's no perceptable runout on the crank, no perceptable radial or axial movement from bad bearings etc. The only thing I haven't checked is endplay on the conrod -- but they certainly feel about the same when rocked around TDC.
I'm using the same fuel, same prop brands, same glowplugs -- there are very few variables not accounted for.
In the next couple of days I'll be reinstalling my SA100 on a plane and running it up again. I might fit some accelerometers to the airframe and build a four-axis vibration map throughout the rev-range. That may give more clues as to what's causing the vibes.