12. Lifter Bores - Lifter bore diameter is one of the most critical dimensions on a block, and occasionally a block will need one or more lifter bores to be sleeved with a bronze guide and honed to size. Moreover, high-lift flat-tappet cams with aggressive lobes can benefit from larger diameter lifters, which require a boring, sleeving, and honing procedure shown here. Paul Higgins, instructor at the University of Northwestern Ohio, tells us that lifter bore diameters vary greatly, even in the same engine family, depending upon when the block was produced. His maximum acceptable tolerance for engines built at UNOH would be in the .010-.012 range.