July '20

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60 THE SHOP JULY 2020 Our block was finish-machined at Gress- man Powersports in Fremont, Ohio. During initial inspection, the CNC digital probe first references the block's main bore centerline, establishing a precise reference. The digital probe measures the raw cylinder bores, obtaining the true centerline and revealing any off-center tolerances. This aids in the boring process to achieve a precise center during milling. PRECISION ENGINE Since our crank total stroke is 3.875 inches, we consider half of total stroke, which in this case is 1.9375 inches. Added to this is our 6-inch rod length and piston compression distance of 1.062 inches, which totals 8.9995 inches. Following digital probe readings on the CNC, decks were milled to a finished height of 9.001 inches. Our piston skirt diameter, measured at the precise point specified by ICON, mea- sured 4.1587 inches. With that in mind, the bores were CNC-milled with a carbide cutter to 4.160 inches initially, leaving 0.0042 inch for final honing. Once all bores were milled, the tops of the bores were lightly chamfered on the CNC center. Providing a light chamfer of the top bore edges eases piston and ring installation by eliminating the sharp upper edges. The advantage of CNC machining, which was not commonly available back in the day when these engines were first in their heyday, involves both precision and repeat- ability. Prior to machining, the block is set up on the CNC machining center, followed by digital probing of existing deck height and squareness, cylinder bore centers and lifter bore centers. If any deviations are present in terms of factory machining or casting shift, instead of assuming that bores are precisely cen- While aftermarket performance blocks are available that accept either a 350 or 400 main bore, I chose the 350 mains to reduce rotating mass. Back to Basics The ICON piston skirts measured 4.1587 inches. Our finished bores at 4.1642 inches obtained a wall clearance of 0.0055 inch. With deck plates in place, cylin- ders were final-honed to size, followed by a single pass with pla- teau brushes to provide better ring seating. Our honing deck plates were torqued to 65 pound-feet, simulating installed heads. This allowed us to obtain a superior cylinder wall roundness during honing, compensating for cylinder wall distortion that occurs when the heads are torqued in place.

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