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If the main bearing bores are not aligned properly, the crank cannot run true, and the bearing loads will be uneven. Main bearing cap distortion can be the result of many cycles of heating and cooling over the lifetime of an engine. Depending upon the amount of correction required to bring the alignment true, either align-boring or align-honing may be necessary. We feel at least align honing should be part of every rebuild on a used or "seasoned" block. If new or different main bearing caps are being installed, align boring is required to achieve both straightness and proper sizing along the main bearing caps.