Condensed Answer: A set of BMX cranks can be installed on a dirt jumper, but the conversion requires a threaded bottom bracket compatible with а 19 or 22mm spindle.
Bottom Bracket Compatibility
The main obstacle stopping the direct installation of BMX cranks on a dirt jumper would be the bottom bracket.
The used bottom bracket should be compatible with the dirt jumper’s frame and capable of accepting 19mm or 22mm spindle/axle. (Most BMX cranks are designed for spindles with a 19mm or 22mm diameter.)
The only BMX bottom bracket that answers these criteria is known as Euro BB.
This type of bottom bracket has other names too – BSA, BSC, ISO, British.
BSA – Birmingham Small Arms (a British industrial combine)
BSC – British Standard Cycle (a British Imperial screw thread standard)
ISO – International Standard Organization
Before buying a bottom bracket, you have to make sure that it’s the right width for the BB shell of your frame.
Most MTB/dirt jump frames have 68/73mm wide bottom bracket shells.
Installing a Euro BB Bracket
Step 1: Determine the Spacing of the Bottom Bracket
This is a fundamental step meant to ensure that the bottom bracket fits the frame perfectly.
If the bottom bracket is too wide, the cranks will feel unstable.
If the bottom bracket is too narrow, the bearing will bind.
Ultimately, you want the cups of the bottom bracket resting nicely against the outer part of the bottom bracket shell.
To ensure, this outcome, one has to search for the right combination of axle sleeve + washers + spacers.
Here’s how that’s done:
1. Slide the spindle through one cup.
2. Slide a sleeve of your choice onto the spindle
3. Add a washer or two.
4. Put the other cup on the spindle.
5. Turn the frame/bike upside down and place the spindle parallel to the wide part of the bottom bracket shell.
If the cups are sticking too far out, remove a washer or use a shorter sleeve.
If the cups are too close to one another, use a longer sleeve or add a washer.
Repeat until the cups rest nicely on the outer lip of the bottom bracket shell.
Note: Don’t proceed to installation until you get this part done. If you do, you may have to remove the bottom bracket and get a new fit.
Step 2: Clean and degrease the bottom bracket shell. Wipe it with a cotton rag and let it dry.
Step 3: Grease the threads and the outer ring/face of the bottom bracket to prevent squeaking.
Step 4: Install one of the cups into the frame. Thread it slowly and use only your fingers without applying excessive force.
You should be able to get the cup down to the outer ring without tools.
Step 5: Push the spindle through the cup. Then slide the sleeve and the needed washer(s) onto the spindle from the other side.
If you use 2-piece cranks begin with the one pre-attached to the spindle.
If you have 3-piece cranks, you can start with either side.
Step 6: Install the other cup.
Step 7: Tighten both cups with a wrench.
Note: The cups tighten in opposite directions.
Step 8: Install the cranks
What To Do If You Don’t Have a Euro BB Shell
If the bottom bracket shell of your frame doesn’t have threading, then it doesn’t support Euro bottom brackets.
This situation leaves us with the following options:
1. Install an adapter
If you have а press fit BB30 bottom bracket, you can convert it to BSA/Euro bottom bracket via an adapter such as FSA’s BB30 to BSA converter.
The adapter is pressed into the bottom bracket after the bottom bracket is installed.
It’s mandatory to apply Loctite 609 or an equivalent installation compound to the adapter and the seating surfaces of the bottom bracket before installation.
Note: Before buying an adapter measure the BB shell of your bike. If it’s 68mm wide, you need a 68mm adapter. If it’s 73mm, you need a 73mm adapter.
You can find FSA’s full installation manual in this PDF file.
There are BSD adapters for PF30 (Press Fit 30) frames too. One example would be SRAM’s PF30 to 83mm Euro BB converter.
Once the adapter is mounted, you can proceed to install a Euro BB bracket designed for your BMX cranks.
2. Bottom Bracket Reducer Adapter
Another option is to use a bottom bracket reducer – an adapter making your existing bottom bracket compatible with spindles of smaller size.
Such reducers exist for both BB30 and PF30 bottom brackets.
3. Use a Race BMX Bottom Bracket
The brand Profile Racing offers a press-fit bottom bracket designed to fit BB30, BB86, and BB92 MTB bottom brackets.
The bottom bracket comes in two versions – for 19mm and 22mm spindles.
What Are The Advantages Of BMX Cranks?
A set of quality BMX cranks is as strong as it gets. They’re made of heat treated 4130 steel a.k.a. Cr-Mo and are practically unbreakable unless you’re a professional rider.
Many riders dig the look of BMX cranks and put them on their dirt jumpers and MTBs for style points rather than strength.
Truth be told, MTB cranks are more than strong enough for most riders’ needs.
3. Shorter Cranks
In some places, shorter BMX cranks are easier to find than shorter MTB models.
If you want to, you can go down from 175mm MTB cranks to 160mm BMX cranks.
The main benefit of shorter cranks is that they are stiffer (shorter moment arm) and allegedly more knee-friendly.
Summary: What You Need To Know
- If your dirt jumper has a threaded bottom bracket shell, you can install a Euro BMX bottom bracket on it. Then, you can easily add BMX cranks to the bike.
- If your dirt jumper has a press-fit bottom bracket, you can use an adapter to turn the existing bottom bracket into a threaded one.