I have discovered when machining rods for insert bearings that the Babbit used by GM in OEM connecting rods is extremely tough stuff. It can dull a high speed steel cutting bit with just a couple of passes. Rods reconditioned by aftermarket suppliers usually have much softer bearing metal than GM used originally. I've started using nothing but tungsten carbide cutters in my micrometer boring head for roughing out the rod bores, then finishing the diameter with a Sunnen hone.
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!