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The coated sleeves are placed into the CNC mill for all the detail cutting and drilling. The sleeves for LS1 engines (Gen III) come in two bore size ranges. The smaller set covers from 4.125 to 4.160 inches, while the lager goes from 4.170 to 4.200 inches. For the Gen IV blocks, there's just one part number that covers from 4.125 to 4.200 inches.