How To Replace a Bottom Bracket On a Bmx Bike

Updated on August 18, 2022

How To Replace a Bottom Bracket On a Bmx Bike

In the days before chassis and disc brakes, home mechanics were terrified of bottom brackets and threaded headsets. The advent of the threadless headset has made headset repair easier than ever before, and the removal of a mountain bike’s bottom bracket has never been easier.

Most mountain bikes still use threaded BB cups, despite the rise of press-fit and conventional BBs in succeeding model years. Shimano’s system is the easiest to set up and use, but if you want to see significant gains in performance and dependability, consider upgrading to a higher-end model. XTR bottom brackets cost ten dollars more than SLX, but in our experience, they last much longer.

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One-Piece to Three-Piece BMX Bottom Bracket and Crankset (2 Techniques)

External bottom bracket replacement is now a reasonably straightforward process, however it’s important to read the manufacturer’s instructions for individual spacer configurations. Measure the diameter of your frame’s shell — either 68mm or 73mm is appropriate (on all but downhill frames). Any spacers that need to be inserted should be easy to locate.

BMX bottom bracket and crank upgrades are covered in this guide.

Method 1: Use a lighter spindle and individual crank arms instead of a single crank and bearings.

Replacement of the single-piece crankshaft with an adaptor and sealed cartridge bearings (method 2). (and separate crank arms).

We’ll start with some more broad inquiries.


There are several parts that make up a BMX bottom bracket.

One of the BB Shells:
Geometrically speaking, a hollow tube at the junction of the key tubes

to the bottom of the page (headtube to BB shell)

tube for seating (houses seat post)

stay-in-places (rear dropout to BB shell)

Bottom brackets are built around the BB shell, or “housing,” if you like.

In terms of frame geometry, the BB shell is the lowest point that supports the most important front, vertical, and rear-facing tubes.


Types of Bottom Brackets for the Bearing Assembly

There are a variety of bearings available.

The most important thing to remember is that each arrangement can accept a different type of crank.

In the most basic configuration, bearing cups are used as bearing races, and bearings are placed inside cages that are positioned in the drive and non-drive side races.
Finally, the BB shell is fitted with seal-tight bearings that are either press-fit or threaded (not common).
Another option is to insert a bearing cartridge into an adaptor that is pressed into the BB shell.

The bottom bracket is an American BMX BB shell adapter fitted on a normal English-threaded square taper bottom bracket.

Three. The Spindle

Also referred to as a “axle.” It’s not, however.

The bearings revolve around a fixed axle. Axles, for example, are a part of every wheel.

The bearing spins the spindle. All bottom brackets feature spindles.



There are three options available to you here: 1.


All-In-One Crankshaft Set

The most basic crankset with caged ball bearings in bearing cups.

The crank and spindle are a single unit, making them hefty and inconvenient.

In addition, changing your chainring is made more difficult by the rig.


Rectangle Taper

Replace your 1 piece crankset with a 150 mm spindle and the same bearing configuration. To put it another way, it’s a three-part crank.

You’ll have a more aerodynamic rig that looks much better.

Even better, you may use a square taper cartridge BB adapter to install sealed bearings that run more smoothly and require less maintenance.


For the first time, the cartridge’s four tapered faces are replaced by splines that connect the spindle to the crank.

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What Size Bottom Bracket Do I Need For My BMX Bike??

All of this relies on the size of the BB shell you have. The inside diameter is what you need to know. To get a better idea of what a bottom bracket is, see the entry on Bottom Bracket Standards.

The OD (outer diameter) of the correct bearing can be determined using this ID measurement.

51 millimeters in the United States

Inches: 41 mm

In Spanish, it measures 37mm.

In millimeters, Euro is 31mm.

If you don’t plan on changing your crank, your bottom bracket options are limited by the sort of crank you currently have.

To operate a different type of crank requires a change in the type of BB.

Toys for Bicycle Bottom Brackets

It’s up to you to decide what equipment you’ll need based on the job at hand. And your crank/bottom bracket set-up makes a difference.

If you’re working in a high-tech environment, you’ll need more specialist tools.

A (chrome vanadium) socket set is a valuable tool in the workshop for a variety of tasks.

As long as you know exactly what sized nuts and bolts you work with, you can purchase the driver and then each socket as needed for your bike work or maintenance.

The set isn’t necessary.

Cranksets in a single piece

This crankset can be worked on without the use of any specialised equipment.

Have two crescent wrenches on hand: one 300mm and one 250mm (depending on where you live, this may be referred to as “shifting spanners”).

To loosen the bearing, you’ll also need a flat-tip screwdriver and a hammer. We’ll get to that in a moment.


Crank Tools with a Square Taper.


Special tools are required here.

Because each can be purchased on Amazon or at your local bike store, there’s no need to worry.

If your square taper rig needs any repairs, the local technician has all the necessary tools.

The cartridge bearing may be installed or removed with ease using this splined drive.

If you don’t have it, you’ll have to go to the bike shop. Without a splined drive, the cartridge cannot be installed or removed.
With a long-handled wrench, it is much easier to tighten the crank bolts.

To ensure that the cartridge bottom bracket can be adjusted to the proper tightness, make sure it fits the splined drive as well.

Socket wrenches are sufficient. However, it’s best to extend the handle in order to get more leverage. Make use of an old seatpost or a piece of tubing.

Remove your cranks with a crank puller. Alternatively, you can obtain them without handles and use a crescent wrench to drive the bolt.


Spindle and Crank Tools with Splines

The crank bolts require the use of a long-handled wrench.

A simple socket wrench would do the trick. Check the bolts for looseness on a regular basis.

The long-handled wrench makes it much easier to correctly tighten them, so consider adding one to your essential tool set.

To loosen and tighten crank bolts, you’ll need a suitable hex drive.



Starting with the bearings, we’ll move on to removing the crank and replacing it with a single spindle and separate crank arms.
If you’d rather watch the video, we go over both of the steps in detail there.

Remove All Links in the Web

Remove the wheel nuts by loosening them with a 19mm socket.

Let go of the left first. The chain tension has now loosened, causing the wheel to pull to the left.

After that, loosen the right side to allow the wheel to slide forward in the horizontal dropouts all the way.

The chain is now free of the chainring, allowing you to remove it.


Single Crank Dismantle

The best tool for this job is an adjustable wrench.
The crankshaft and bearing assembly are held together by a locknut and a bearing ring.
Locknuts and bearing rings can be accommodated by the adjustable wrench.
Remove the bearing ring from the lock ring by loosening and removing the washer.

The screwdriver tip should be inserted into the slot. To remove the ring by hand, simply tap the ring a few times with your finger.

The entire assembly comes apart when you screw the bearing ring along the crank and spindle.


Remove the old bearing and bearing ring from the crankshaft.

Next, insert the crank and the remaining bearing into the bottom bracket shell and tighten the bolts on both.

Keep an eye on the chainring while you’re working.

The left side bend of the crank must be manoeuvred through the bottom bracket shell without causing any damage to or impact on the frame.

It doesn’t matter whether your frame is ancient (and the paint job has faded). However, you don’t want to damage any remaining paint or decals.

Installation of BMX Bottom Brackets with Spindle and Bearings Kit.
Prior to installing the square taper spindle, we’ll show you how it works.
The press-fit adapter H515 from First Components is used in the second technique.

The adapter is inserted into the bottom bracket shell, and a cartridge bottom bracket is then inserted into the adapter to complete the installation.


Bottom Bracket Conversion Kits are discussed in detail in another post.)

Mechanics of the Spindle-and-Bearer Assembly

You put two bearing rings on each side of the spindle thread.

As a bearing race, the cages rest on top of the bearings, and the cups rest on top of the bearings.

To keep the assembly together and the bearings attached to the race, tighten the bearing rings.

These bearings will last for many years if they are properly lubricated and maintained.

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Preparation of Bearings and Spindles

Installing a bottom bracket kit like this one.

Following that, I’ll explain how everything works and the recommended course of action.

Start by lubricating the threads on the spindle’s drive side.
Attach the spindle to the bearing ring with a screwdriver.
To remove any old grease and dirt from the bearing cups, wipe them down with a damp cloth.

So, even if the bearing set comes with new cups, there’s no need to remove and reinstall the old ones.

However, we’ll be using the kit’s bearings instead.


Apply a new coat of grease to the cups once they have been cleaned.
Grease the bearing liberally. Fill the void between the bearing cage and each individual bearing with grease.
When you put the spindle’s bearing ring on, apply some grease.
The left, non-drive side bearing ring and spindle thread should also be lubricated.

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