What size is the crankshaft nut?
There is no universal crankshaft nut size; it varies by engine and model.
In this article, we explain how to determine the exact size for your specific vehicle, why sizes vary, and how to replace the nut safely and accurately.
Why the size varies
Crankshaft pulleys and their securing nuts are designed to fit the specific mounting pattern, thread size, and torque requirements of individual engines. Factors such as engine family, crankshaft diameter, pulley design, and the presence of a locking feature or a washer all influence the correct nut size and type. As a result, a single “one-size-fits-all” answer doesn’t exist across makes, models, and years.
How to determine the correct nut size
To identify the correct nut size for your crankshaft pulley, follow these practical steps:
- Consult the vehicle’s service manual or the manufacturer’s official website for the exact nut size, thread specification, and torque specification for your engine.
- Identify your engine code or VIN and search for the specific service bulletin or torque table that references the crank pulley nut.
- If the manual isn’t available, inspect the existing nut and pulley: measure the nut across the flats (AF) with a caliper or a ruler to estimate the hex size, and measure the thread pitch and diameter by comparing with a thread gauge or using a thread-diameter tool.
- Take the old nut or the pulley to an auto parts store or a machine shop to verify the correct size against their reference catalogues or sockets.
- When replacing, ensure you also have the correct washer, locking device (if used), and the specified torque. Do not reuse damaged or worn hardware.
Following these steps helps ensure you select the correct nut and torque specification for a safe, reliable installation.
Common sizes you might encounter
The following lists summarize typical measurements you may encounter when shopping for a crankshaft nut. Use them only as a general guide, and verify against your vehicle’s official specifications.
Note: The sizes below refer to hex across flats (AF) and common metric thread ranges found on automotive crankshaft nuts.
- Across-flats (hex) sizes you might see: 22 mm, 24 mm, 26 mm, 28 mm, 30 mm, 32 mm, 36 mm (larger diesel or heavy-duty applications).
- Common metric thread sizes for the nut threads: M12x1.5, M14x1.5, M16x1.5, M18x1.5, M20x1.5, M22x1.5 (others can occur in specialty engines).
In all cases, rely on the official torque specification and any locking mechanism recommended by the manufacturer. If in doubt, consult a certified technician or the dealership service department for your exact engine.
Notes on torque and installation
After identifying the correct nut size and thread pitch, use the exact torque specification prescribed by the engine designer. Many crank pulley nuts are secured with high torque values and some require a locking washer or a thread-locking compound approved by the manufacturer. Improper torque can lead to pulley wobble, belt misalignment, or in severe cases, engine damage. Always follow the OEM guidelines and use the proper tools, including a calibrated torque wrench and, if needed, a torque angle gauge for final tightening.
Additional considerations
Some engines use a locking or anti-rotation device (such as a split lock washer, a tab washer, or a locking nut) and may require a specific sequence for tightening or a separate tightening procedure during maintenance. If your maintenance involves replacing the timing belt or chain along with the crank pulley, ensure all components are reinstalled according to the service manual.
Summary
Crankshaft nut size is engine-specific and cannot be generalized across all vehicles. Determine the correct size by consulting the service manual, identifying engine codes, and verifying thread diameter and hex size with measurement tools or professional help. Always apply the manufacturer’s torque specification and use any locking hardware as required. By following these steps, you ensure a secure fit and reliable engine operation.
