Compression Bolts

BMX compression bolts: how headset preload is set

Despite where it sits in the catalogue, this part has nothing to do with the drivetrain: it fits at the top of the fork steerer and exists purely to compress the headset stack — cup, bearings, spacers, stem — and remove play before the stem is clamped. Once the stem is tight, the compression bolt does no more work: it never holds the fork in.

What a kit consists of

• The top cap: the dished cap that bears on the top of the stem or on the last spacer.
• The bolt: usually an M6, sometimes M8 on reinforced kits, in steel, 7075 aluminium or titanium.
• The anchor: a star nut driven into the steerer, or an expander whose cone is drawn up by the bolt itself.

Star nut or expander: it depends on the steerer

On an aluminium or steel steerer, a conventional star nut is driven a few centimetres below the top of the tube and holds perfectly. On a carbon steerer, a star nut is out of the question: its teeth create stress points that start a crack. There you need an expander, whose cone presses a collar against the full inner circumference and spreads the load. If you are unsure what your steerer is made of, do not force anything and ask us to confirm.

Sizes, drives and materials

BMX race kits are designed for a 1"1/8 steerer, 28.6 mm outside diameter, by far the dominant standard. The drive is generally a 5 or 6 mm hex, or a Torx T25 on recent models: Torx transmits more torque and is much less likely to round out than a hex in aluminium. On materials, steel is the most forgiving, titanium saves weight without becoming fragile, and anodised aluminium is mainly there to match the colours of a build — on an alloy bolt, care with tightening is essential.

Setting preload correctly

First slacken the stem clamp bolts on the steerer completely. Then tighten the compression bolt gradually until play disappears: hold the front brake and rock the bike back and forth, and no knocking should be felt in the head tube, while the fork should still swing freely under its own weight with the bars released. Finally line the stem up with the front wheel and torque its bolts to the recommended figure. Compression bolt torque stays very low, well below stem bolt torque: tightening it further gains nothing and crushes the bearings.

When to replace it

A rounded drive, a damaged thread, a split top cap or a star nut that pulls up the steerer as you tighten all call for immediate replacement of the whole kit. It is an inexpensive part whose failure leaves you with headset play you can never take up, and therefore bars that knock on every landing. Lightly grease the thread on reassembly, especially with a titanium bolt running into an aluminium expander.

The BMX RACING advice

We stock kits from several brands and can point you to the right one based on your steerer material and the height of your spacer stack. Apply the same care with torque across the whole bike as you would on your BMX cranksets, and browse all our drivetrain parts to group your order. Dispatched within 24 to 72 hours and you can spread the cost over 2 or 3 payments with Alma.