THE SHOP

July '20

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62 THE SHOP JULY 2020 The lightweight forged ICON pistons weighed in at a mere 406 grams. PRECISION ENGINE tered, a computer program compares the readings to block specifications. In the process, for example, if cylinder bores are, say, 0.0005 inch off-center, the boring operation will follow the intended bore centerlines, correcting any off-center issues (if they exist). When the decks are milled, this not only achieves the desired deck height from the main bore centerline to the deck surfaces, but establishes a true parallelism of the decks with the main bore centerline, with equal dimensions on both right and left bank decks. It simply doesn't get any better than this. With CNC machining now being much more available among upper-level machine shops, the entire process of machining and correcting for cylinder bore, lifter bore and deck dimensions can be accomplished with a high degree of precision and time-savings, without the need to move the block among different machining centers. Once decking and boring operations are complete, the block simply moves to a honing station for surface completion of the cylinder walls. CYLINDER HONING Once all CNC milling operations were com- pleted, Gressman moved the block to his Peterson honing station, where honing plates were secured to each deck. The use of honing plates, also called deck plates, simulates the distortional effect of installed cylinder heads. While you may think that a block is rigid enough to prevent cylinder wall movement, in reality even the strongest blocks can experience slight cylinder bore distortion when the cylinder heads are installed. The honing plates mimic the installed state, allowing honing to be more precise to provide a more uniform bore in a fully assembled condition. The cylinders were honed starting with 200-grit diamond stones to a final dimen- sion of 4.1642 inches. In order to obtain a piston-to-wall clearance of 0.0055 inch, final cylinder wall finish was accomplished with four light passes with 500 stones, fin- ishing with one pass with plateau honing brushes, to establish a more uniform sur- face finish and minimize microscopic peaks and valleys for superior oil retention and to aid in faster piston ring seating. Each bore received a single pass with the plateau brushes. This essentially cleans the high points left by the honing while leaving enough of a surface profile to allow the piston rings to properly seat. Our raw block's lifter bores initially mea- sured 0.9040 inch. Our Morel solid roller lifters measured 0.9038 inch. Morel speci- fies clearance at 0.0015 to 0.0018 inch, so Gressman milled the bores by 0.0014 inch to size all lifter bores at a finished diameter of 0.9054 inch to achieve oil clearance of 0.0016 inch. Back to Basics After correcting for the bobweights, the crank's static and dynamic balance was extremely close to zero, thanks to the efforts of balancing whiz Jody Holtrey of Medina Mountain Motors in Cres- ton, Ohio. The forged and lightweight SCAT crank fea- tures a 3.875-inch stroke and 2.100-inch rod journals.

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