Good point Phil,
These early six cylinder engines with the three bolt pulley attachment were more suseptable to problems. A crankshaft pulley/vibration dampner assembly which comes loose can ruin the crankshaft. I know many of these engines with AC which never had problems and I know some that did. It is most critical that everything be correctly assembled on the M127 (230-SL) engine when installing the multiple crank pulley. Be sure to correctly torque the crankshaft bolt and the three pulley bolts. The correct strength hardware should be used. The factory shop manual reccomends using some thread locking compound (Locktite or such). Modifications and upgrades to the fastening hardware and re-assembly in detail is covered in section 03-10/1 to 03-10/4 of the Workshop Manual "Passenger Cars Starting August 1959" This section covers vibration dampners and counterweights on six cylinder models from 220 b to 250-SL. It looks like there is an improved crankshaft "strech bolt" and washers which can be used to prevent problems. I can scan the information for you if you do not have it.
Joe Alexander
Blacklick, Ohio