A line bore is where they actually "bore" the main caps straight. Usually during a rebuild they hone through the mains. A block requires boring when you install different main bearing caps. For example, a Chevy two bolt main cap block has two bolts per cap. You can upgrade these to a four bolt style after drilling and tapping the block. Then the block needs to be bored to make sure all caps are perfectly aligned. I helped put together a motor where the guy used "splayed" main caps-meaning the outer bolts of the four bolts were angled rather than straight. I guess this would distribute the load more evenly. Anyway, he didn't want to bore it. After the caps were barely tightened down, you couldn't turn the crank anymore. Guess what?? It was pulled back apart and line bored. Go figure.
