The later-model 216's had insert main bearings, but it was possible to fine-tune the clearance with shims. On the 235's and 261's it was just a holdover from the previous manufacturing procedures, since millions of similar engines had been made that way.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!