How to Replace the Bottom Bracket on a Mountain Bike!

Approx Reading Time: 18 minutes

The most common problem among mountain bikers is the wear and tear of the bike’s basic parts. These parts consist of a long list including tires, wheels, and most importantly the bottom bracket. The bottom bracket is a combination that holds the most significant parts of the crankset and allows the conversion of linear motion to rotary. If you don’t know how bottom brackets work and the issues related to it.

How to Replace the Bottom Bracket on a Mountain Bike:

  • Step 1: Crank Removal
  • Step 2: Bearing Decompression
  • Step 3: Safety Catch Removal
  • Step 4: Extracting The Crank
  • Step 5: Determining Thread Type
  • Step 6: Dust & Grease Removal
  • Step 7: Attach the Bearing
  • Step 8: Chainset Removal
  • Step 9: Reassemble

The bottom bracket of a mountain bike is situated in the inner layer of the frame with the help of the bottom bracket shell. This shell provides a durable figure for the crankset and gear mechanism involved with the pedal. There are a lot of bearings associated with this piece and these bearings can wear out eventually and need replacement.

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Replacing a bottom bracket is a pretty tricky piece of work. There are a lot of things you need to know before even starting an attempt if you want to do it yourself. Otherwise, all the custom dealers and bike shops have repair facilities ready at any time. But, if you want to treat your baby the right way, you must do the work yourself.

Throughout this article, I’m going to share all I know about bottom brackets from my 25 years of mountain biking experience. Just sit back, hold on and read through the whole thing to get a clear view on what to do and what to avoid. Let’s start!

Types of Bottom Brackets

If you’re planning on removing or repairing or even replacing your bottom bracket, the first thing that comes is the type. There are mainly two types of bottom brackets in the market these days. One of them is the “Threaded” bottom brackets and the other one is “Press-fit” bottom brackets. Here’s some detailed info on these two types! In case you don’t have time to go through the whole characteristics, here’s a table containing the basic features of threaded and press-fit bottom brackets!

CharacteristicsThreadedPress-fit
Requirement of ThreadsRequiredNot required
Connection TypeConnects both the arms using clockwise or anti-clockwise rotationConnects both the arms using only anti-clockwise rotation
ComponentsMost common types are the three-piece threaded typesMost common types are the two-piece types
RemovalRemoval is not easy, requires many tools and precise indicationRemoval is easy compared to the threaded types
InstallationInstallation needs correct knowledge and a lot of practiceInstallation is as easy as the removal
Diameter SizeCan be of large diameter than the internal frameCan not be larger than the frame diameter
Bearing PositionBearings are situated outside the shell of the bottom bracketBearings stay inside the shell of the bottom bracket

Threaded Bottom Brackets

The name suggests that it has something to do with threads and it does. These types of bottom brackets usually come with a shell. The shell is connected with internal threads that work as a holding mechanism for the parts associated. 

The shell can be either directly connected with the frame through threads. Or, it can be inserted through the help of an external part that is connected after the main part is active. Either way, threaded shell bottom brackets have a tight hold on to the parts integrated into the main bike frame. 

The history of threaded bottom brackets comes with different surprises at different times. There are many standards that have been maintained over time. Nowadays, there are bottom brackets with different width, length, and diameter. The most common standard size maintained among manufacturers is 35mm diameter, 24 threads, and 1.37” width. According to the English standard, the non-drive or mostly the left side comes with the right hand and the other one is left-hand threading.

Press-fit Bottom Brackets

Press-fit bottom brackets are also attached to the main bike frame, but there are no threads to seal it. A simple bore like part is used that serves as the connection between the frame and the shell. The bearing cup in this is a little larger in diameter than the actual bore of the bike’s frame.

The shell is tightly pressed all the way inside that creates an airtight seal between the shell and the bore. The bottom bracket stays at its place because of this tight fit. Although it all may seem pretty easy, the manufacturers have made press-fit bottom brackets a bit confusing for the users. Some of the models in the market are named according to the diameter of their spindle, while others were numbered according to the width of the shell and there are more. 

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The only constant thing is the bore diameter of the bike and bikers throughout the world buy parts according to the size of the bore. And, throughout this article, I’m going to constantly refer to this terminology. PF41 is the most common and popular press-fit model that has a 41mm diameter.

How To Determine a Worn Out Bottom Bracket

You might never know when the right time is to give a look at your bottom bracket. Of course, there are certain processes to find out if your bottom bracket needs some attention or not. One of the most effective processes is looking for the noise.

After running for months, the bearings start to lose their outer shell, resulting in much more friction than usual. If this friction continues, the outer layer slowly and gradually looses its natural smoothness and the friction gathers at a certain period. Because of this increased friction, the energy associated is converted into sound and you can easily hear it or feel it while you’re riding. Listen closely and try to feel your bike as it is a part of your body.

When the bearings are worn out, they create a rumbling noise that can be felt and if the wear is too much, the noise can be even heard. If your bike is making a noise like that, its high time you changed the bottom bracket.

Now that you have found out the need of replacing your bottom bracket, here comes the hard part. To make the best use of resources and replace them in the correct way, follow these steps accordingly. We’re going to start with the removal of the old bottom bracket and move on to installing a new one later. Let’s start then!

Replacing a Threaded Bottom Bracket

Step 1: Crank Removal

Cranks work like levers with the sole purpose of connecting the main spindle with the pedals. Toothed chainrings are closely fitted into the crank that keeps the chain of the bike at the correct place and pace. To replace the bottom brackets, removing the crank is the first thing you must handle. But be advised for there are consequences to your actions!

The crank system was pretty easy in the past but as bikes got more advanced and modern, crank systems updated with it too. Nowadays, the crank system used in a low-profile mountain bike is far better than a high-spec bike 20 years ago. To exactly pinpoint the safe removal of your crank system, you must know the types of cranks found in the market. The type of your crank system will be given via the manual provided by the manufacturer. But if you can’t figure out the correct crank system of your bike, here’s a list of the most common types of crank systems!

Self-extracting Crank System

This crank system consists of a crank puller that is perfectly fitted into the crank. They come with threaded rings. These rings are closely threaded with the bolt. When it is loosened little by little, the shoulder gives a firm push against the outer layer and results in the removal of the arm from the inside. You won’t need any additional supplies of tools to remove this type of crank.

Compression Crank System

This type of crank system comes with a two-piece feature. There are two crank arms- left and right. The left one has a slot that is compression enabled and held together through two bolts. While the right one comes with a pre-attached spindle. This crank system does not need any extra tool because it comes with bearing cups.

Three-Piece Crank System

The three pieces of this crank system are the left and right arm and main spindle. The arms are joined together and they keep the spindle at its place through the help from bearings. Both the left arm and right arm must be removed before removing the bearings of this system. For your ease, there are bolts at both the arms to remove the cranks safely.

One Piece Crank System

This is a rather old type of crank system found in older models of mountain bikes. It comes with a single piece of the arm that connects the crank to the pedal from both sides. This single part works as the main bearing angle for the bottom bracket.

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In a nutshell, to remove the crank, you must get rid of the bolts from both sides. You can use a simple bolt loosening tool according to the bolt size. Normally, a 5mm pinch key would do the work. But, if your crank has bigger bolts, use a key with a larger diameter. Be careful of the force you give to loosen up the bolts. They are pretty tightly attached and due to friction, loosening them might be tricky. Try to use safe lubrication if there’s too much rust at the bolts. If you put too much force on a rusty bolt, the tip of the bolt might get broken and you will have a lot of trouble getting the rest of the bolt out.

Step 2: Bearing Decompression

The next step for you is decompressing the bearings attached to the bottom bracket. For this, you must first locate the compression cap attached to the pedal and crank. With the help of a special tool, like the Park Tool BBT, you can easily remove the cap.

While removing the cap, remember to do it slowly and know your strength. Putting too much pressure on the cap might get it in a difficult condition. Despite using blunt force, try to get a firm grip over the tool and find the right angle of rotation.

Step 3: Safety Catch Removal

On both sides of the crank, there’s a small button like figure attached to the pedal between the bolts. This is called the safety catch. The safety catch is an option created to hold the pedal in its place and avoid unnecessary breakdown of the crank.

To disengage the safety catch, take a flat-head screwdriver and push the catch upwards a little. Again, there’s no need to prove your strength here. Just a little push at the right place will do the work perfectly. After successfully disengaging the safety catch, you will easily be able to slide the crank arms outwards. The arms will come off smoothly after the safety is off. If you’re facing issues with disengaging the catch, give it a quick clean and try again.

Step 4: Extracting The Crank

 If the bottom bracket is suffering from too much wear and tear, it will be difficult to extract the crank from the frame. To safely extract the crank, get the chain off from the smaller ring very carefully. The chainring should be kept at ease on top of the bottom bracket shell. 

Hold the exact point where both the rings join together at the crank and gently pull or push the assembly out of the bottom bracket. If normal force doesn’t work and rings are not smooth for transition, apply an opposite force or use a lubricant to ease up the material for removal.

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Step 5: Determining Thread Type

There are two standards of threads out there in the market. One is the American standard and the other one is the Italian standard. You can get rid of the bottom bracket easily with a bottom bracket tool that works with the spanner.

To follow the American or British standard, you need to rotate the right-hand arm in a clockwise direction in order to remove the arm. For the left arm, an anti-clockwise rotation is required.

On the other hand, in Italian standards, both the arms need to be rotated in an anti-clockwise direction to unhook. The direction is always marked on the side of each of the arms. Look at your crank closely to understand in which direction you need to rotate.

Step 6: Dust & Grease Removal

Due to extreme usage, the threads seem to wear out or dip in a lot of grease and dust. After removing the crank successfully, the most important thing is to make sure you clean the dust and grease on the threads. Squeaky clean the edges especially with a simple brush and water-solvent mixture. Both the sides and edges need to be smooth enough to easily and tightly fit the new bottom bracket.

If the threads stay dirty, it will affect your new bottom bracket and wear out faster than you anticipate. After installation, if the bottom bracket wears out too quickly, be sure it’s because of the treads. If the thread is beyond the condition of your expertise, maybe it’s time you paid the mechanic a visit. Refurbish the threads with appropriate cutting and smoothening tool.

Step 7: Attach the Bearing

After you’ve successfully removed the old bearing, cleaned the threads and crank, it’s time for fitting the new equipment. If you haven’t bought the new bottom bracket yet, look at my detailed classification of bottom brackets to find the one you need.

There should be a new sleeve in the fresh bottom bracket. Attach this to the right side of the bottom bracket cup. Use your fingers only to hook this to the sleeve that’s attached to the internal frame. When you can’t move the part with your finger anymore, attach the left bearing to the opposite side in the same manner.

If you already have a spanner for the bottom bracket, it’s high time you used it. When both the left and right arm are at their place and finger tight, use the spanner to tighten them accordingly. Don’t put too much pressure as there will be increased wear and tear with the increase in friction.

One of the most common mistakes that people do here is cross-threading the arms. It’s pretty easy to mess the whole thing up by going in through the wrong threads. If you feel the threads are wrong when you’re attaching by your hand, just simply take it out and try to roll it inside again. Don’t just use brute force while inserting the arms.

Step 8: Chainset Removal

For the next step, we’re going to remove the old chainset and replace it with the new one. First, take out the chain from the bottom bracket. Take the right side and push it out of the bottom bracket. There are chainrings of increasing diameter by the side. Keep the chain at the smallest chainring.

Try out the chain with a gentle spin. If the chain is running smoothly without any interruption or noises, keep it as it is. Now that you’re done with the right side, get the left side out through a push from the opposite side. This way, both the sections will be ready to go.

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Step 9: Final Touch

After you’ve successfully attached the bottom bracket, it’s time to re-attach the compression cap you removed at the first step. Take a new compression cap and tighten it with your fingers. Be careful not to make it too tight as it will increase friction and the bearing will wear out pretty quickly because of it.

Give the crank some spin to test if you’re successful or not. If there’s any type of problem with the installation, you’ll hear creaking noises. Otherwise, you’re good to go!

Replacing a Press-fit Bottom Bracket

Replacing a press-fit bottom bracket is far easier than replacing a threaded bottom bracket. Here’s how you can do it!

Step 1: Getting Ready

A press-fit bottom bracket has no threading or any type of connection with the internal frame. There’s a pretty smooth surface where you need to fit the replacement piece. Get a drift set to take the bearing out. Remember to clean the surface with cleaning agents twice before attempting to connect a new one.

To clean a frame made from aluminum, using grease is a smart choice. Grease can keep the surface smooth and allow smooth transitions. It can make the smoothness available to the rider for a long time. There’s a wide debate though, on whether you can use grease on Carbon Fiber bikes or not. But as I’ve seen many bikers do it and still face no issues, I would advise everyone to use it.  

Step 2: Ready Your Cartridge

The cartridge of a normal press-fit bottom bracket comes with two different pieces that can be installed from both sides. Both of these parts consist of bearings and connect with the sleeve. There are probably a hundred types of press-fit cartridges out there in the market, but the mechanism is the same for almost all of them. You can install any one of them by simply following this method.

The next thing is to attach the cartridge in its place. To do that successfully, you need the correct set of tools. There’s a net amount of pressure that must be exerted on the cartridge in order to perfectly fit it in its place. The Park Tool BBP-1 may come in handy in this process. The press must connect itself with the outer surface in order to avoid getting damaged.

Step 3: Perfect Fit

The adaptor needs to be correctly fitted to the press and it must be loaded to the bottom bracket cup. Make this assembly outside and when you perfect it just push it inside the internal frame. Create the adapter for the other side, in the same manner, push them on the other side.

Remember to keep the handle stretched out so you tighten the press correctly. Tighten the press according to your need and after you’re done with tightening it, push it as far inside as you can.

Step 4: Finishing Touch

Check if all the connections are properly tightened. Try to tighten with your hand and feel if there are any voids or creaking noises. Fit an appropriate Allen key right at the backside of the press. If you connect the cups and tighten it accordingly, you’re done!

Remember not to put too much pressure on the fitting part. The press-fit type bottom brackets don’t require brute force to be attached to the frame. You just need to give a simple push and the work will be done. Don’t try to overdo by giving a demonstration of your strength. Believe me, your bike doesn’t want to see your muscle power.

Maintenance of Bottom Bracket

Most of the bikers out there spend all their time biking through busy roads and going on long rides. But, what they all lack is the culture to take care of their bikes properly. If perfectly taken care of, the bearings and bottom bracket can last more than 5000 miles. But it depends completely on the willingness of the rider.

You can’t always wait for a creaking noise to happen in order to open up or service your bike. Regular maintenance can keep your bike in shape for a much longer time frame. Here’s how you can take care of your bottom bracket properly!

Open It Up

Don’t hesitate to remove the total bottom bracket from the internal frame. There was a rumor that once you remove the bottom bracket of your bike, it can never be assembled back. That’s complete rubbish. Bottom brackets are meant to be opened up and serviced regularly to be in shape.

Clean Regularly

If you’re a commuter and use your bike to get around the neighborhood, open up your bike and clean the bottom bracket and inside the frame at least twice per month. I would have suggested washing once per week, but everyone’s too busy nowadays to do that.

And, if you’re a frequent traveler and love to go on long rides, wash your inner parts like the bottom bracket and crankset once per ride. You can wash the parts after you finish the ride or before you go on your next ride.

Grease Helps

There are many people out there who would advise you not to use grease because they think it makes your bike slow. But I would strongly advise you to use grease at the contact points of the bottom bracket even if you have a carbon fiber bike.

Using grease increases the shelf life of the parts of a bike. Grease increases the smoothness between two surfaces that results in less wear and tear than usual, even in higher speed variations.