6. Icon 428 forged pistons are in the 421. While a stock 421 had a bore of 4.097 inches, the 428 bore was 4.120. These pistons are a 0.030-inch 428 overbore, resulting in a dimension of 4.150 inches. The high-ductility 2618 alloy pistons are fully machined and feature Diamond-finished skirts, dual forced pin oiling, top-land mini-groove technology, and second-land accumulator grooves, and come with triple-wound spiral locks.