MTB Topbolts

What is an MTB top bolt and what is its main purpose?

In MTB, this is often referred to as the Ahead Top Cap. Its main function, together with a Star Nut driven into the steerer tube, is to adjust the headset to be play-free before the stem is tightened.

What are the differences to BMX bolts?

Star Nut System

MTB forks usually do not have a welded internal thread. You must drive a star-shaped nut into the steerer, which the bolt grips into.

Top Bolt System (BMX-Style)

Some rigid street MTB forks use an internal thread (e.g., M24) like BMX bikes. No star nut is needed here.

What variants are there?

• Standard Top Caps: Simple caps made of aluminum or carbon, often with a logo to visually finish the cockpit.
• Hollow Bolts: Essential for Dirt Jumpers who do barspins. These bolts have a hole in the center through which the front brake cable can be routed down through the steerer tube to the brake.

How do I install a top cap?

1. Ensure the star nut sits straight and approx. 15mm deep in the steerer tube.
2. Install the stem. The steerer tube should end approx. 2-3mm below the top edge of the stem.
3. Place the top cap on and tighten the bolt hand-tight with an Allen key.
4. Check for play: Pull the front brake and rock the bike. Nothing should move inside the head tube.
5. Only now tighten the side bolts of the stem!

What do the specifications mean?