Cummins 4089963 Piston Assembly Kit

The Cummins QSL9 piston assembly utilizes a two-piece articulated design consisting of an aluminum alloy crown and a forged steel skirt connected by a hardened piston pin. This configuration manages thermal expansion and mechanical stress inherent in high-pressure common rail (HPCR) and CAPS fuel systems operating at peak cylinder pressures exceeding 180 bar. The kit includes the piston crown, piston skirt, piston pin, piston pin retainers (snap rings), and a complete ring set comprising two compression rings and one oil control ring. Critical dimensions such as compression height, bowl geometry, and ring groove profiles are machined to OEM tolerances to ensure proper squish clearance and combustion efficiency. The piston pin bushing (part 3942087) is serviceable separately. This assembly is calibrated for engines equipped with CM850, CM554, and CM2250 electronic control modules. Installation requires verification of connecting rod small end bore diameter and piston-to-cylinder wall clearance per Cummins service manual specifications. The component is a direct replacement for OEM part numbers 4941395, 4955237, and 4941393 across QSL9, QSL8.9, ISL, QSC, and ISLE engine platforms.