Changing the bottom bracket on a mountain bike: How to replace the bottom bracket on your bike
Jan Timmermann
· 12.02.2026
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Replacing the bottom bracket on a mountain bike is not rocket science and is a must, especially for frequent riders. Depending on the seal, load and environmental conditions, a standard set can be worn out after just 2000 to 3000 kilometres. Unfortunately, the industry has not yet been able to agree on uniform dimensions and standards. So before replacing them, mechanics must first find out which spare part they need.
In principle, the exact designation of the respective bottom bracket standard should be obtained from the frame manufacturer. A distinction is made between bottom brackets whose shells are pressed into the chassis and those that are bolted. The thread is more user-friendly, but if all the parts involved are of high quality and the right tools are used, a pressed bearing can also be replaced in around 15 minutes. The diameter, or the thread, of the bottom bracket must match the housing and the inner diameter must match the crank axle. In modern parts, the latter is part of the crank, not the bearing, but is also not standardised. For example, a Shimano crank does not fit into a bottom bracket with a Sram DUB axle standard. Of course, there is also a suitable tool for removing and installing each bottom bracket.
To make matters worse, a bottom bracket must also harmonise with the width of the housing. If you are unsure, you can use a caliper on the frame to measure the width. For most mountain bikes, a width of 68 or 73 millimetres has become established, for which one type of bearing often fits together. Exceptions can be found on downhill bikes and individual models with a Superboost rear wheel axle. The following tables provide an overview of the most common dimensions:
Bolted bearing shells
| Type | Inner diameter |
| BSA | 34 mm |
| T47 | 46 mm |
Pressed bearing shells
| Type | Inner diameter |
| BB 86/92/104/107/121 | 41 mm |
| BB 30 | 42 mm |
| PF 30 | 46 mm |
Crank axle
| Type | Outer diameter |
| Shimano Hollowtech II | 24 mm |
| Sram DUB | 28.99 mm |
| various manufacturers | 30 mm |
How to change the bottom bracket
It is not without reason that we also rate the serviceability of the bottom bracket on every bike in the BIKE test system. However, the actual replacement of the bottom bracket is child's play once you have identified the right spare parts and tools. We explain the replacement of bolted and pressed bottom brackets in three simple steps.
Replacing the bolted bottom bracket
1. pull off the crank: Almost all modern cranks have an integrated removal function. In the case of the Shimano parts on our example bike, the two clamping screws on the non-drive side must first be loosened. The main screw can then be removed using a special Shimano tool and the locking plate lifted with a small flat-blade screwdriver. The left crank arm is now detached. The right-hand side can be knocked out by lightly tapping the axle with a plastic hammer.
2. remove the old bearing: There is a special spanner or a suitable nut (e.g. BSA or T47) for each bolted bottom bracket housing. Caution: The right-hand bearing has a left-hand thread, the left-hand one a right-hand thread. The shells can therefore be loosened in the opposite direction to the direction of travel. There are usually one or more spacer rings between the cups and the frame to correct the chain line. It is best to make a brief note of the number on each side so that you can refit them correctly.
3. install a new bearing: The threads and inside of the frame should be thoroughly cleaned before refitting. Apply assembly grease to the threads of the new bearings and reinsert the spacer rings (for 68 mm housing two on the right, one on the left / for 73 mm housing one on the right, none on the left). Then tighten the bearing shells with the torque specified by the manufacturer (around 40 Nm) in the direction of travel. The contact surfaces of the crank axle should also be cleaned and regreased. Remove excess grease after fitting the crank.
Replace pressed bottom bracket
1. knock out the old bearing: Once the crank has been removed (see above), pressed bottom brackets can be hammered out of the frame using a knock-out tool. Different tools work in different ways, but careful but firm blows are always advisable. Without special tools, it is advisable to go to a specialist workshop, as the frame can be damaged during removal. Pressed bearings also use plastic parts, the position of which should be memorised during disassembly.
2. press in new bearingThen clean and re-grease the housing. With most pressed bottom brackets, it does not matter which bearing is fitted on which side. A press-fit tool suitable for the bearing diameter must now be inserted through the housing. When pressing in, make sure that the bearing sits straight in the frame at all times. Finally, remove the tool, insert any plastic parts and reinsert the crank after cleaning and regreasing.
3. the right twist: Especially with pressed bearings, the smooth rotation of the crank is sensitive to tolerances in the frame. Many of the current crank models have a ring on the non-drive side for adjusting the bearing preload. To do this, loosen the clamping screw after tightening the crank arms, turn the ring until the crank rotates smoothly but is still seated without play and carefully tighten the clamping screw again.
Jan Timmermann
Editor
