Ditto to all the above. The most common reason for line boring is if one of the main bearings fails and "spins" in the block during a catastrophic engine failure. If the scoring of the block (not the cap) can be smoothed out by removing a couple of thousandths of material, the block can be salvaged. If it requires a deeper cut than that, it's probably junk. I begin a line bore by getting the boring bar exactly centered in the bores in the block, up and down and side to side, and then carefully move the bar toward the top of the block about .002" (the thickness of a strand of hair). Then a .010" cut is made at the parting line of the main bearing caps, shortening the top bottom dimension of the bore. The line bore cutter is set to cut the hole back to the original diameter, and all the main bearing bores are cut before the bar is moved. This takes .002" out of the block, and .008" out of the caps. This results in the holes being in a straight line and the correct diameter, with the crankshaft centerline .002" closer to the top of the block. This induces a very slight bit of slack in a timing chain, or it moves the teeth of a gear-driven cam and crankshaft closer together. As long as the teeth of the timing gears don't bottom out with each other, no problem! On a racing engine, the engine buildup begins with a line bore, and all the other dimensions like cylinder boring and block decking, etc. are indexed off the crankshaft centerline.
Jerry
