A Cummins engine overhaul kit (OHK) is key for restoring engine performance, containing gaskets, seals, bearings, pistons, liners, and hardware. To select the right kit, identify the exact engine model and serial number, determine the overhaul scope (major, in-chassis, or gasket set), and choose between genuine Cummins or premium aftermarket kits from reputable sources, ensuring compatibility and quality for a successful rebuild.