When you bore a cylinder, you’re scraping a thin layer of metal from the wall of the cylinder. There are a few different reasons you’d want to bore a cylinder. One is to remove damage such as pitting or rust. Another is to increase the horsepower of the engine by allowing for a bigger piston. As long as attention is paid to how carefully it is done and certain constrictions, boring a cylinder is an accepted practice in both auto repair and performance shops. First, and most important is that you bore all of the cylinders the exact same amount to balance the power output among cylinders. Second, is that usually the most you will want to bore a cylinder without installing a sleeve, is .060 inches. This is known as boring it 60 over.