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To offset-grind the crank is to take material off one side of the rod journal to fit the big end of a smaller-than-stock rod. That moves the centerline of the rod journal further from the mains-that's where we get the extra stroke. Mike Johnson at JMS did a bit of counterbalance work as shown here. The Chevy-style rod and forged piston weighed about 590 grams; 150 lighter than stock.