How to Install the Fox 38 Air Shaft Assembly

By FOX Canada | Published: 2026-10-23

Category: How-to Guides

Step-by-step instructions for replacing the air shaft on your Fox 38 fork, including common mistakes to avoid and a look at the Fox 38 Air Shaft Assembly (2021-25').

Quick answer

To install the Fox 38 Air Shaft Assembly, you need to remove the fork from the bike, drain the oil, separate the lowers, swap the air shaft, and reassemble with fresh oil. The Fox 38 Air Shaft Assembly (2021-25') is a direct replacement that fits all Fox 38 forks from those model years.

Why replace the air shaft?

Over time, the air shaft seals can wear, causing air leaks or reduced suspension performance. Replacing the air shaft restores proper travel and air-spring feel without buying a whole new fork. The Fox 38 Air Shaft Assembly (2021-25') is designed as a direct swap, so you keep your existing damper and lowers.

Tools and supplies you'll need

You'll need a shock pump, 24mm socket, 8mm hex wrench, T25 Torx bit, pick tool, clean rag, and FOX 20wt suspension oil (or equivalent). A fork oil level tool is helpful but not required. Work in a clean area to avoid contaminating the internals.

Step-by-step installation

Remove the fork from the bike: Loosen the stem bolts, remove the front wheel, and unthread the brake caliper (hang it aside). Loosen the pinch bolts on the lower crown and slide the fork out. Be careful not to pull on the brake hose. Support the fork upright to prevent oil spill from the damper side.
Drain the oil and separate the lowers: Remove the air valve cap and depress the valve to release all air. Remove the 8mm hex bolt at the bottom of the fork leg (air spring side). Slide the lowers off the stanchions. Catch the old oil in a drain pan. If the bottom bolt is stubborn, use a T25 Torx through the top cap to hold the shaft while loosening.
Remove the old air shaft: Pull the air shaft out of the upper tube. Use a pick to remove the wiper seal and foam ring from the lower leg. Clean the inside of the upper tube with a rag. Do not scratch the inner surface of the upper tube; use a plastic pick if possible.
Install the new Fox 38 Air Shaft Assembly: Lightly grease the outer seal of the new air shaft. Slide it into the upper tube until it seats fully. Reinstall the foam ring and wiper seal. Apply a small amount of oil to the wiper seal lip. Ensure the air shaft is fully seated; a gap means it isn't installed correctly.
Reassemble the lowers and add oil: Slide the lowers back onto the stanchions. Install the bottom bolt with a new crush washer (if supplied). Pour the recommended volume of 20wt oil into the air spring leg (typically 10-15cc, check your manual). Overfilling can cause lockout or harsh bottom-out; measure carefully.
Reinstall the fork and set sag: Mount the fork back on the bike, tighten all bolts to spec, and reinstall the wheel and brake. Pump the fork to your desired pressure, then cycle it a few times to equalize. Set sag per your riding weight. After the first ride, recheck the air pressure and bottom bolt torque.

Product used in this guide

Fox 38 Air Shaft Assembly (2021-25'): The Fox 38 Air Shaft Assembly (2021-25') is available in multiple travel options and fits Fox 38 forks from 2021 to 2025 model years. This means you can choose the exact travel you want (e.g., 160mm or 170mm) and install it without modifying other fork components.
Fox 38 Air Shaft Assembly (2021-25')