Topbolts

What is a Topbolt and what is its main purpose?

The Topbolt (also known as Compression Cap or Fork Bolt) screws into the top of the fork steerer tube. Its primary function is to pull the fork upwards against the frame to preload the headset bearings. It eliminates play (wobble) in the front end. Once the stem is tightened, the Topbolt technically has no structural load, but it keeps the system secure and covers the steerer tube.

How is a Topbolt constructed?

Cap (Head)

The top part that sits on the stem or spacers. Often countersunk or flat, featuring the tool interface.

Thread / System

Either a large threaded shaft that screws directly into the fork (High-End) or a bolt that screws into a pressed-in star nut (Entry-Level).

Tool Interface

Usually a 6mm or 8mm Allen key hole, sometimes a square drive for a ratchet.

What types of Topbolts are there?

Threaded Topbolts (Integrated)

The standard for high-quality aftermarket forks. The fork has an internal thread (e.g., M24 or M25). The Topbolt screws directly into the steerer tube. While many brands share standards, they are not automatically interchangeable. Always check for compatibility (thread size and pitch) or stick to the fork manufacturer to be 100% sure.

Star Nut System

Common on entry-level complete bikes or in BMX Race and MTB. A steel star nut is hammered into a threadless steerer tube. A longer, thinner bolt pulls a separate cap down to compress the headset.

Design (Hollow vs. Solid)

Some threaded bolts are hollow to allow a front brake cable to pass through. Others are solid for a cleaner look.

What materials are used?

7075-T6 Aluminum

High-strength aluminum alloy. Very light and durable. Standard for high-end threaded bolts.

6061-T6 Aluminum

Slightly softer, used on standard forks.

Steel

Used for Star Nuts and the corresponding tension bolts on cheaper forks for maximum durability at a low price.

Which Topbolt is right for my needs?

The choice depends 100% on your fork model.

• Threaded Forks: Check the manufacturer's description (e.g., M24 x 1.5 or M25). Measure the thread diameter if unsure (24mm vs. 25mm).
• Threadless Forks: If your fork has no internal thread, you need a Star Nut kit (1-1/8" size).
• Cut steerer? Ensure the Topbolt doesn't bottom out on the internal threads before compressing the headset. You might need spacers.

How do I install a Topbolt?

1. Loosen Stem: Ensure the side bolts of your stem are loose.
2. Grease: Apply a little grease to the thread.
3. Tighten: Screw the Topbolt in. Tighten it until the play in the headset is gone, but the handlebars still turn freely. Do not overtighten!
4. Secure Stem: Align your handlebars and tighten the stem's side bolts.

How do I identify and fix common issues?

• Headset still loose: Your steerer tube might be too long (sticking out above the stem). Add a spacer so the cap can compress the stem down.
• Stiff steering: The Topbolt is too tight. Loosen stem bolts, slightly loosen Topbolt, re-tighten stem.
• Stripped thread: Aluminum threads damage easily if forced. Always screw in by hand first.

What do the specifications mean for Topbolts?