I have never gotten a satisfactory answer to a certain question. The term "main" bearing has always been a mystery.
Does it mean the bearings in question are the primary support bearings, as opposed to the secondary support bearings? Are they drilled to provide an oil flow to lubricate the piston rods? Is it a combination of the two? Or does the term have no meaning whatsoever?
The mains are the largest bearings in the engine which hold the crankshaft to the engine block. That alone tells us where the term main bearing came from.
Each bearing is made up of two half moon inserts. The inserts located in the block have oil passage holes that line up with the oil feed holes from the engine block. The other inserts fit into the "main caps". The main caps are heavy cast iron components which bolt down to the engine block to secure the crankshaft between the two bearing inserts. This enables the crankshaft to rotate freely while being lubricated by the incoming oil pressure.
There are also oil passages traveling through the crankshaft. The oil starts at the oil pump and goes through the engine block to the main bearings and "main crankshaft journals" (journals are the machined surfaces of the crankshaft that ride on the bearing inserts). After lubricating the mains, the oil travels through the crankshaft passages and to the connecting rod bearings and journals.
The connecting rods are the parts which connect the pistons to the crankshaft. The pistons are driven downward in order to rotate the crankshaft, by the explosion commonly referred to as internal combustion.