BMX Guide
How to Build a BMX Bike from Parts
Build a BMX from parts by checking compatibility first, then fitting the headset, forks, bars, bottom bracket, cranks, wheels and chain in a set order.
Updated 5 min read1,006 words
To build a BMX from parts, first confirm that the parts fit each other, then assemble in a set order: headset and forks, stem and bars, wheels, bottom bracket and cranks, chain, pedals, seat and brake. The checks that cause the most trouble are the bottom bracket and crank spindle, the headset and head tube, the rear hub width against the frame dropouts, and chain line. Get these right before buying anything.
Compatibility checklist
Run through this list before you buy. A mistake here means returned parts.
| Check | What must match | Where to look |
|---|---|---|
| Bottom bracket to frame | The frame's BB shell type must match the BB (for example American, Mid or Spanish) | Frame spec, bottom bracket |
| Crank spindle to BB | The spindle diameter (commonly 19 mm or 22 mm) must match the BB bearings | Cranks spec |
| Headset to head tube | Integrated, semi-integrated or external cup headset must suit the head tube | Frame spec, headset |
| Forks to headset | The steerer tube must be the right diameter and long enough | Forks |
| Rear hub to dropouts | Hub width must match the frame's dropout spacing, and the axle must fit the slots | Cassette hub or freecoaster hub, frame spec |
| Front hub to fork | Hub width and axle size must match the fork dropouts | Front hub |
| Stem to bars | Bar clamp diameter (usually 22.2 mm for BMX) must match the stem | Stem, handlebars |
| Stem to steerer | Stem bore must suit the steerer diameter | Stem and fork spec |
| Seatpost to frame | Post diameter must match the seat tube | Seat and seatpost |
| Brake to frame and rim | Brake mounts and the brake style must suit the frame and rim | Brakes |
| Chain line | Sprocket and rear cog must line up | See below |
Always read the manufacturer's spec rather than guessing. When in doubt, ask the shop before buying.
Tools you need
- Allen keys: 4, 5, 6 and 8 mm cover most BMX parts
- Spanners or a socket set for axle nuts and pedals
- Bottom bracket tool that fits your BB
- Crank puller or the correct tool for your crank type
- Headset press or a careful alternative for pressing cups in
- Chain breaker
- Tyre levers and a floor pump
- Grease, thread locker if the manual recommends it, and rags
- Torque wrench (helpful)
- A bike stand or a way to hold the frame steady
Order of assembly
Follow this order. It keeps jobs from getting in each other's way.
- Prepare the frame. Clean the threads in the bottom bracket shell, the head tube and the dropouts. Grease them lightly.
- Fit the headset. Press the cups into the head tube if it uses pressed cups. Install the crown race on the forks. Keep the parts in the right order. See headset.
- Install the forks. Slide the steerer through the head tube, add spacers, the stem and the top cap. Leave it slightly loose for now.
- Fit the stem and bars. Centre the bars, set the angle and tighten the clamp bolts evenly.
- Install the bottom bracket. Fit the BB for your frame type. Use grease and the correct tool. Do not cross-thread it.
- Fit the cranks and sprocket. Slide the crank spindle through, align the arms at 180 degrees, and tighten the crank bolts. Fit the sprocket and check it for wobble.
- Fit the wheels. Place the front wheel in the forks. Place the rear wheel in the dropouts, slide it to the middle of the slots and tighten the axle nuts finger tight.
- Fit the chain. Wrap it around the sprocket and cog. Cut it to length with the chain breaker and join it with a master link. Set tension by sliding the rear wheel back. See how to fix a BMX chain.
- Fit the pedals. Right pedal clockwise, left pedal anticlockwise. Grease the threads.
- Fit the seat and post. Set the height and angle and tighten the clamp.
- Fit the brake and cable. Install the lever, cable, caliper and optional gyro. Adjust the pads.
- Fit the grips. Slide them on with a little hairspray or grip glue, then add bar ends.
- Inflate the tyres. See BMX tyre pressure.
- Final checks. Check every bolt, spin the wheels, test the brake and take a short test ride.
Bottom bracket and spindle in more detail
The bottom bracket is where mistakes are most common. BMX frames use one of a few BB shell standards, and cranks use a spindle of a set diameter. The BB must suit both the shell and the spindle. For example, a frame built for a Mid BB needs a Mid BB, and the spindle size you choose must match the bearings inside it. Do not assume a 22 mm spindle fits a BB made for 19 mm. Check the product pages. See bottom bracket for the main standards.
Rear hub and dropout width
Rear dropouts are normally a standard spacing, but not always. Measure the gap between the frame's rear dropouts, then check the hub's overall width. If the hub is wider than the gap, the frame will not fit. If it is narrower, spacers may be needed. Also check that the axle diameter fits the slots in the frame and that the axle nuts or bolts match. A freecoaster hub or cassette hub both follow the same fitting rules.
Chain line
Chain line is how straight the chain runs between the sprocket and the cog. A bike with good chain line has the sprocket and cog lined up on the same vertical plane. To check, look down from above. The chain should run parallel to the frame. If it angles, adjust with spacers behind the sprocket or the hub. A crooked chain wears the sprocket and may fall off.
After the build
Ride slowly for the first few minutes, then recheck every bolt. New builds settle in quickly. Use BMX maintenance basics as your regular routine, and read how to set up a new BMX for bar angle, seat and brake setup.
Related
Frequently asked questions about How to Build a BMX Bike from Parts
Is it cheaper to build a BMX from parts?
Usually not. Complete bikes are priced lower than the same parts bought separately. Building makes sense when you want exact parts, a particular fit or a bike you can upgrade over time.
What tools do I need to build a BMX?
A set of Allen keys, spanners or sockets, a bottom bracket tool, a headset press or a careful alternative, a crank puller, a chain breaker, tyre levers, grease and a torque wrench if possible.
What is chain line?
Chain line is how straight the chain runs between the front sprocket and the rear cog. A straight chain runs quietly and wears slowly. A crooked one rubs, skips or falls off.
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