I’ve always wondered this. Take a look at the photo in the link below. See how some of the smaller holes that go through the top of the block are round? And some, the larger ones, 4 each surrounding a cylinder in this case, are not round? These holes in question – I presume they are for the coolant as they are the largest holes – these holes are sort of an oblong “D” shape.
I can understand how the engine manufacturer could make a round hole in a block with a drill, but how do they make the “D” shaped holes?