Bontrager Expander Plug and Top Cap Headset

Protect your carbon steer tube the right way. The Bontrager Headset Expander Plug and Top Cap is the correct and recommended alternative to a traditional star nut for Trek carbon fibre steer tubes — offering a secure, damage-free installation that won't compromise the structural integrity of your fork.

Unlike a star nut, which is hammered into place and can cause irreversible damage to a carbon steer tube, this expander plug uses a clamping mechanism that spreads load evenly across the tube interior. It installs cleanly into all Trek 1-1/8" carbon fibre steer tubes and is approved for use on all Trek carbon fibre 1-1/8" forks. The included top cap completes the headset stack for a tidy, finished look.

Please note: this expander plug is not a replacement for a star nut on aluminium steer tubes and should not be used as such. Always install one spacer above the stem when using on carbon steer tubes.

Key Features:
- Safe, secure alternative to a traditional star nut for carbon steer tubes
- Compatible with all Trek 1-1/8" carbon fibre steer tubes
- Expander clamp design — no hammering, no damage to carbon
- Includes top cap
- Inner expander bolt torque: 9–10Nm
- Top cap bolt torque: 4Nm
- Not for use on aluminium steer tubes in place of a star nut