The late-model 518 is a prime example of this statement. When we get OE cores, the covers measure 0.160 to 0.165 inch thick if they are undamaged. The front covers can normally be cleaned up by machining off 0.005 inch of material. The challenge comes when we get a cover that was galled or damaged by a major lockup failure. In these instances, you may have to machine off as much as 0.015 inch, and at this point you end up with a cover that is only 0.145 inch to 0.150 inch thick. The question is, how far can you go? Where is the line between thick enough and no longer thick enough?