The Fox 40 Factory suspension fork is a top-tier component designed for aggressive downhill racing and freeride applications. With its precise engineering and high-performance damping system, maintaining this fork is critical to ensure optimal function and longevity. This guide covers the complete process of rebuilding the Fox 40 Factory, including disassembly, inspection, cleaning, reassembly, and tuning. It also addresses common setup issues, compatibility considerations, and performance optimization tips.

Tools Required

• 10mm, 32mm socket and torque wrench [paid link]
• 2mm, 5mm, and 6mm hex wrenches
• Soft-faced hammer
• Suspension oil (Fox 20wt Gold and 5wt Teflon)
• Seal driver set for 40mm stanchions
• Clean rags and isopropyl alcohol
• Nitrile gloves
• Torque wrench [paid link]
• Shock pump
• Fox 40-specific dust wipers and foam rings
• Plastic pick for seal removal

Disassembly and Inspection

Begin by thoroughly cleaning the fork exterior. Remove the fork from the bike frame and release any air pressure from the air spring using the shock pump. Loosen the lower leg bolts using a 5mm hex wrench and tap them lightly with a soft-faced hammer to disengage the shafts from the lowers.

Once the bolts are free, slide the lowers off the fork legs carefully to prevent damage to the bushings. Drain the bath oil and inspect it for contamination. Check the foam rings, dust wipers, and internal seals for wear, tears, or deformation. Replace any parts that show visible damage.

Examine the stanchions for scratches or scoring, especially near the wiper seals. Any deep damage can cause seal leakage and premature wear, requiring repair or replacement of the stanchion.

Cleaning and Preparation

Thoroughly clean all internal surfaces using a lint-free rag and isopropyl alcohol. Ensure that all old oil, dirt, and residue are removed from the lower legs and the damping assembly.

Install new foam rings soaked in Fox 20wt Gold oil, and press-fit new dust wipers using a 40mm seal driver. Apply a light film of suspension grease to the wipers and install the foam rings underneath them. Grease the air spring assembly and damping shaft if you are servicing these components simultaneously.

Reassembly of the Fox 40 Factory

Slide the lower legs back onto the stanchions carefully, ensuring that the foam rings and wipers remain in place and undamaged. As the lowers engage with the stanchion tubes, ensure both damping and spring shafts align correctly with their respective ports in the lower leg.

Inject the correct amount of 20wt Gold oil into each lower leg. Fox recommends 30cc for each side of the 40 Factory model. Torque the bottom bolts to Fox specifications (usually 5.7Nm), ensuring even and secure closure.

Pressurize the air spring to the recommended pressure for your weight and riding style, typically between 70–120 PSI. Cycle the fork several times to distribute the oil internally and confirm smooth travel.

Troubleshooting Common Issues

If you experience excessive stiction or harshness after the rebuild, verify the following:

• Ensure proper lubrication of the foam rings and seals.
• Double-check that the dust wipers are seated correctly and not pinched.
• Confirm correct oil volume in the lower legs.
• Inspect the damper and air spring assembly for full engagement.

Persistent oil leakage may indicate damaged bushings, a scratched stanchion, or incorrect seal installation. Address each issue by isolating the cause through careful reinspection.

Customization and Performance Tuning

The Fox 40 Factory fork allows riders to adjust high-speed and low-speed compression, rebound, and air spring pressure. For maximum performance:

• Tune high-speed compression to control big-hit absorption.
• Adjust low-speed compression to fine-tune cornering and braking support.
• Set rebound damping to balance responsiveness with traction retention.

Add or remove air volume spacers to change spring progression. More spacers will make the fork feel more progressive, while fewer will result in a linear feel. This is useful for customizing the ride feel for specific terrain or riding styles.

Compatibility Considerations

The Fox 40 Factory fork is compatible with both 27.5″ and 29″ wheels, depending on the specific chassis configuration. Ensure rotor size matches the fork’s minimum and maximum rotor diameter support. Most models support up to 203mm rotors.

When pairing with drivetrain components, ensure adequate fork crown clearance and cable routing, especially when using integrated handlebar and stem systems. The 40 Factory is typically used with DH drivetrains like SRAM X01 DH or Shimano Saint.

Always verify headset and steerer compatibility before installation. The Fox 40 uses a standard 1.125″ steerer tube with an external lower crown race.

Comparison with Similar Models

The Fox 40 Factory stands above its peers in the high-end suspension fork segment, particularly when compared to models like the RockShox Boxxer Ultimate or Manitou Dorado Expert. Key advantages include:

• Kashima-coated stanchions for reduced friction.
• Grip2 damper system with wide adjustability.
• High stiffness-to-weight ratio for aggressive terrain.

However, riders looking for lower maintenance intervals may consider the RockShox Boxxer for its simpler servicing procedure. Those seeking inverted designs might favor the Manitou Dorado, though it may lack the damping refinement of the Grip2 system.

FAQs

How often should I rebuild my Fox 40 Factory fork?
Fox recommends a lower leg service every 30 hours and a full rebuild, including damper and air spring inspection, every 100 hours.

Can I use aftermarket seals in the Fox 40 Factory fork?
While some aftermarket seals are compatible, it is best to use genuine Fox parts to ensure proper fit and performance.

What air pressure should I run in my Fox 40 Factory fork?
Air pressure varies based on rider weight and style, but typically ranges between 70–120 PSI. Use the Fox tuning guide for specifics.

Why is my Fox 40 Factory fork losing travel?
Loss of travel may be due to air transfer issues or a stuck negative spring. A full air spring service may be required to resolve this.

How can I reduce harshness over small bumps?
Ensure your fork has the correct oil volume and that foam rings are properly lubricated. You may also want to reduce compression damping slightly.

Can I convert my 27.5″ Fox 40 to a 29″ setup?
No, conversion is not recommended due to differences in chassis dimensions and axle-to-crown height. Use the correct version for your wheel size.