Why are there lockwashers on the main bearing bolts in the first place? Maybe a hardened steel SAE flat washer might be OK, but it's the torque on the bolt, not a lock washer, that keeps main bearing, rod bearing, and head bolts tight. Split-type lock washers in particular have a nasty habit of spreading out under heavy torque, and allowing a bolt to loosen itself. Star-shaped washers create metal shavings that don't need to be in the crankcase.
Jerry
