MTB Headset Top Caps & Star Nuts

Top cap and star nut: what they actually do

These two small parts are not decorative. The star nut is pressed permanently into the fork steerer; the top cap threads into it and pulls the stem down. It is this combination that preloads the headset. Once adjusted and the stem bolts tightened, the top cap no longer bears any load: its job is done.

Star nut or expander plug

• Star nut (star nut): driven permanently into a steerer tube using a dedicated tool aluminium or steel. Economical and permanent — it cannot be pulled out, only driven further down.
• Expander plug (compression plug): expanding wedge assembly tightened inside the steerer tube. Mandatory on a carbon steerer, where a star nut would crack the material. Repositionable, and therefore reusable after recutting.

The top cap

• 1"1/8 diameter in almost all cases.
• It is tightened gradually and never overly tight : a few newton-metres are enough, since it does not hold anything once the stem is clamped.
• The bolt can be replaced on its own if it is rounded or seized: no need to replace the whole assembly.
• Shapes vary: a flat cap clears the view of the cockpit, while a taller cap is easier to access with a hex key.

The correct installation order

Spacers in place, stem slid on with bolts loose, top cap tightened gradually until all headset play is eliminated, then tighten the stem bolts to torque. If you tighten the stem first, the top cap can no longer remove the play: this is the most common reason for a headset coming loose after every ride.

When to replace it

• Top cap thread damaged, or rounded bolt head.
• A star nut that spins inside the steerer or pulls up when tightened: it no longer grips.
• Changing fork: the star nut stays in the old steerer, it cannot be recovered.
• Switching to a carbon steerer tube: you must replace the star nut with an expander plug.

To find out more: How to choose your MTB top cap, star nut and spacers

See also: MTB headset spacers · MTB stems · MTB headsets