Headset top caps and expander plugs

Star nut or expander plug: the choice depends on your steerer

These small parts are used to preload the headset : screw the top cap into the anchor to preload the bearings, then tighten the stem to hold everything in place. There are two types of anchor, and they are not interchangeable.

• Thestar nut is a finned metal insert that is driven in by force into the steerer. It is permanent, deforms during installation and cannot be refitted. Only for aluminium or steel steerers.
• Theexpander plug slides in easily, then expands against the inner wall when tightened. It can be removed and repositioned, and it distributes the pressure. It is the only device suitable for a carbon steerer.
• Driving a star nut into a carbon steerer will split it, sometimes immediately, sometimes a few rides later. This is not a matter of preference.

The measurement to check

It is the steerer inside diameter, not the stem’s. 1"1/8 (28.6 mm outside diameter) covers the vast majority of bikes; 1"1/4 is found on some frames. Some expander plugs come in short or long : the long version increases the contact area and reduces stress on a thin carbon steerer or a steerer cut short.

The top cap

It closes off the steerer and holds the preload bolt. Once adjusted, it bears no load — the stem holds everything in place. A broken or lost top cap can therefore be replaced without mechanical urgency, but it does keep water out of the steerer. Some models are magnetic or incorporate an accessory mount.

Installation, step by step

• Tighten the top cap gradually until the play disappears, without overtightening: you are not clamping the bearings, just bringing them into contact.
• Align the stem with the wheel, then tighten the steerer clamp bolts to the specified torque.
• Check for play again with the wheel held still: if it still clicks, adjust the top cap before touching the bolts.
• For a carbon steerer, an torque of 5 to 6 Nm for the stem bolts is a maximum, not a target.

What else you’ll find here

A few stem and handlebar covers specific to integrated cockpits (Vision Metron), as well as replacement bolts. These parts are model-specific: order them using the exact part number.

To find out more: How to choose a road bike stem

See also: Road bike headsets and bearings · Headset spacers · Road bike stems