Important Tip: The area right around the edge of a threaded hole can "ant hill" upwards when a fastener is tightened. This "ant hill" can mean only the very edges of the hole are pressed against the the bottom of the bolt head (or motor lug) and are in contact with the surface. Champfering the edges of the hole a little means the contact patch is moved outward from the hole edge making for more actual contact area. Any ant-hilling occurs inside the chamfer where it cannot interfere. Carbon steel. 1/2", 5/8" and 3/4".