Cummins 6CT Flywheel Housing — OEM 5401230 / 3908799 for 8.3L & 8.9L Diesel Engines

This flywheel housing (also called an engine adapter housing or SAE bell housing) is an OEM Cummins part with cross-reference numbers 5401230, 3908799, 3906691, and 3902565, designed for the Cummins 6CT 8.3L and 6L 8.9L inline-6 diesel engines. This is an SAE #2 bell housing configuration with a 447.68mm (17.625-inch) pilot bore and an SAE-standard 12-bolt mounting pattern — the industry-standard interface for medium-duty truck, bus, construction equipment, and industrial power unit transmissions. The housing is cast from high-grade ductile iron (not gray iron) because the 6CT engine produces peak torque of 1,100-1,350 N·m, which places the housing under significant torsional stress where it bolts to the engine block rear face. The SAE #2 pilot bore must be concentric to the crankshaft centerline within 0.13mm total indicated runout — a specification maintained by machining the housing bore and engine block dowel pin holes in a single setup at the factory.

Flywheel housing installation on a Cummins 6CT requires the engine rear main seal housing to be installed and aligned first, as the flywheel housing pilots off the seal housing’s outer diameter. The housing-to-block bolts (typically M12×1.75 Grade 10.9) must be torqued to 110-120 N·m in a cross-pattern sequence starting from the dowel pin bolts. The critical measurement after installation is the SAE pilot bore concentricity relative to the crankshaft — this requires a dial indicator mounted on the crankshaft flange with the indicator tip contacting the housing pilot bore. Rotate the crankshaft 360°; maximum allowable total indicated runout (TIR) is 0.25mm. If the TIR exceeds this specification, the housing must be shifted using eccentric dowel pins (available in 0.13mm, 0.25mm, and 0.38mm offset sizes) until the bore is concentric. Common failure symptoms: transmission input shaft seal leaking within 500 hours of installation (housing misalignment causing shaft runout), clutch engagement shudder that changes with engine temperature (housing face not perpendicular to crankshaft), and starter motor pinion jamming against the ring gear (starter mounting bore worn oversize from repeated starter replacements).

This housing fits Cummins 6CT 8.3L and 6L 8.9L engines in truck, bus, construction, and industrial applications. Verify the SAE housing size (SAE #2) and your existing housing casting number (5401230, 3908799, 3906691, or 3902565) before ordering. For related engine components, visit our Forklift Hydraulic Transmission section.

Technical Specifications

• Part Numbers: 5401230, 3908799, 3906691, 3902565
• Engine: Cummins 6CT 8.3L / 6L 8.9L Inline-6 Diesel
• SAE Size: SAE #2 (447.68mm Pilot Bore)
• Material: Ductile Iron Casting
• Bolt Pattern: 12-Bolt SAE Standard
• Max Allowable Runout: 0.25mm TIR

Compatible Applications

Each housing is inspected for pilot bore concentricity and face flatness, packed in reinforced packaging, and ships within 1-2 business days. 12-month warranty. Browse our Forklift Hydraulic Transmission catalog.