You may have noticed the RB26 block has OEM Annular groves in the block side of the main bearing journal, the RB30, and non turbo RB25’s RD’setc do not. They can be easily milled in so that you can run an RB26 main baring shell as all RB Main pin sizes are the same.

The RB26 Main bearings have drilled upper shell to facilitate more efficient main bearing lubrication, the annular grove feeds the oil holes in the back of the bearing shell then a sub annular grove in the bearing shell itself has a free flowing and larger oil disbursement across the larger wider section of the crankshaft pin oil squish (upper side), which in turn prepares the thrust side (bottom of thecrankshaft) with a more efficient oil feed.

Technically the oil is the bearing, the bearing is a platform for the the oil to do its “bearing work”, the crankshaft floats in oil and should theoretically never touch the bearing shell while running.