Designed for Pro Mod style engines, these pistons are specifically designed for nitrous applications. The precision CNC machined rings grooves are designed for .043 back cut, 1/16, 3/16 rings. Compression heights figured at zero deck clearance with listed block height. These pistons will only fit Big Chief cylinder heads. Includes: Pin #990-2930-18-51S (174g) Double spiro locks (#990-042-CS) Vertical and lateral gas ports for maximum top ring seal. Bore: 4.580 Stroke: 4.250 Rod: 6.385 Deck: 9.790 Price per piston.