The term “251” refers to the SAE housing size (SAE #1) but specifically the flywheel mounting pattern. The ISX uses a heavy dual-mass or solid flywheel with a specific bolt circle. The 251mm housing is standard on most over-the-road ISX engines (Peterbilt, Kenworth, Freightliner). Getting the flywheel torque wrong is dangerous, but confusing it with the flexplate torque (used in motorhomes or industrial pumps) is catastrophic.
Do not confuse this with:
Ensure you are using the correct bolt: Cummins P/N 5266322 or 3683610. Do not reuse old bolts. These are TTY; they stretch permanently. torque spec for flywheel bolts on isx cummins 251 updated
| Component | Torque | Notes | | :--- | :--- | :--- | | ISX Flywheel Bolt (251) – Standard | 265 lb-ft | Wet, 3 steps | | ISX Flywheel Bolt (251) – TTY | 265 lb-ft + 90° | Do not reuse | | Flywheel Housing to Block (251) | 44 lb-ft | M12 bolts | | Starter Motor Mounting Bolts | 65 lb-ft | Loctite 242 | | Clutch Pressure Plate (if equipped) | 55 lb-ft | Cross pattern |
Last Updated: [Current Year]
If you are rebuilding a Cummins ISX (Industrial, Severe, eXperience) engine—specifically the high-horsepower variants often referred to under the “251” platform (referencing the 251mm or 10-inch flywheel housing)—getting the flywheel bolt torque wrong is not an option. A loose flywheel can destroy the crankshaft, crack the rear gear train housing, or even exit the bellhousing at high RPM.
In this updated guide, we provide the exact torque specifications, updated sequence, lubrication requirements, and critical technical bulletins for the Cummins ISX 251 flywheel bolts. The term “251” refers to the SAE housing
The ISX flywheel uses a simple cross pattern (star pattern). Do not go in a circle.
A: Over-torquing (exceeding 300 lb-ft or 120° angle) will stretch the bolt beyond yield. The bolt will snap the next time you engage the clutch at 1,800 RPM. Ensure you are using the correct bolt: Cummins
An essay on this torque spec would be incomplete without warning about the consequences of deviation:
These bolts secure the bellhousing to the back of the engine block.