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The CNC program machines an offset cylinder chamfer for a 4.100 bore. This 60-degree offset chamfer unshrouds intake and exhaust valves, similar to the Boss 302 and 351 Cleveland. Most aftermarket heads are 4.100 bore, as are the head gaskets. A 4.030 bore block without this chamfer leaves a wall for air and fuel to get around on the side of the valve. This subtle machine operation is one of the horsepower enhancements available with a fully CNC-blueprinted block.