Parts guide
BMX Three-Piece Crank Upgrade: Will It Fit Your Bike?
Three-piece cranks are the single biggest upgrade for an entry-level BMX. Here is how to check your frame, what comes in the kit, and what else you’ll need.
- By
- Collective, London
- First published
- Updated
- Reading time
- 2 minutes
Most entry-level BMX bikes come with one-piece cranks: a single bent steel bar through the frame. They are heavy, they flex and they bend. Three-piece cranks (two arms on a splined axle) are stronger, lighter and what every serious BMX runs. Our kit is built to fit most BMX bikes, not just ours.
The key is the bottom bracket shell, the tube at the bottom of the frame where the cranks go through. Here is how to check yours.
Chapter 01
Check your bottom bracket shell
The one measurement that decides it.
BMX frames use one of four bottom bracket shells. Measure the inside diameter of the shell with the old cranks out.
| Shell | Inside diameter | Where you find it | Our kit |
|---|---|---|---|
| American (US) | about 51mm | Most bikes with one-piece cranks | Yes, with the included Mid-to-US cups |
| Mid | about 41mm | Most modern BMX frames | Yes, fits straight in |
| Spanish | about 37mm | Some older and pro frames | No |
| Euro | threaded | Some frames, often with external bearings | No |
If your bike has one-piece cranks now, it almost certainly has an American shell, so the kit fits with the Mid-to-US cups. Not sure? Send us your frame model.
Chapter 02
What’s in the kit
Everything between the frame and the pedals, except the sprocket.
- Two crank arms
- Splined axle
- Sealed Mid bottom bracket, with spacers
- Mid-to-US bottom bracket cups
- Sprocket pinch bolt
Reuse your sprocket if it fits the axle, or fit a new one. Our 25T alloy sprocket fits 22mm axles, and 19mm axles with the top hat spacer.
Chapter 03
Pedals: 1/2 becomes 9/16
The bit people forget.
One-piece cranks take 1/2 inch pedals. Three-piece cranks take 9/16 inch. So when you upgrade, you need new pedals. Our 9/16 inch BMX pedals are the budget option, and PX1 alloy pedals come in five colours.
Chapter 04
Fitting
A bike shop job if you don’t have the tools.
You’ll need to remove the old cups and bearings, press in the new cups and bottom bracket, set the spacers so the cranks spin freely with no play, and tighten the crank bolts properly. If you haven’t done it before, any bike shop can fit the kit quickly.
Chapter 05
Questions
Will the crank kit fit any BMX?
It fits frames with a Mid or American (US) bottom bracket, which covers most BMX bikes. It does not fit Spanish or Euro shells.
Do I need new pedals?
Yes, if you are coming from one-piece cranks: three-piece cranks use 9/16 inch pedals.
Does the kit include a sprocket?
No, it includes a sprocket pinch bolt. Reuse yours or fit a new one.
Is it worth it on a cheap BMX?
It is the upgrade that makes the biggest difference: stronger, lighter, and it opens up the whole range of 9/16 pedals.
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