--- title: "Bottom Bracket Explained: Which One Does Your Bike Have?" description: Learn how to identify your bike’s bottom bracket, compare BSA, T47, BB30, PF30, BB86, and BB92 standards, and find the right fit for your frame and crankset. image: https://blog.zinn.bike/hubfs/bottom-bracket-explained.webp --- - [clydesdale bicycles](https://www.zinn.bike/bikes/clydesdale-bicycles/) - [ZINN Custom Bikes](https://www.zinn.bike/) - [Tui Bikes](https://www.zinn.bike/bikes/tui-bikes/) - [303-499-4349](tel:303-499-4349) - [Login or Create account](https://www.bigandtallbike.com/myaccount.asp?__hstc=248507544.90448e05af5b6838a0b3ffd1e40c765f.1752502478952.1753346888326.1753362879282.6&__hssc=248507544.2.1753362879282&__hsfp=945770823) - [View Cart](https://www.bigandtallbike.com/message.asp?msg=49&__hstc=248507544.90448e05af5b6838a0b3ffd1e40c765f.1752502478952.1753346888326.1753362879282.6&__hssc=248507544.2.1753362879282&__hsfp=945770823) [![Zinn logo white](https://blog.zinn.bike/hs-fs/hubfs/Zinn%20logo%20white.png?width=196&height=72&name=Zinn%20logo%20white.png)](https://www.zinn.bike/) [![Zinn logo 200px wide for checkout page](https://blog.zinn.bike/hs-fs/hubfs/Zinn%20logo%20200px%20wide%20for%20checkout%20page.png?width=196&height=72&name=Zinn%20logo%20200px%20wide%20for%20checkout%20page.png "Zinn logo 200px wide for checkout page")](https://blog.zinn.bike/) - [Home](https://www.zinn.bike/) - [Contact Us](https://zinncycles.com/contact/) Open main navigation Close main navigation - [Home](https://www.zinn.bike/) - [Contact Us](https://zinncycles.com/contact/) # Bottom Bracket Explained: Which One Does Your Bike Have? TECHNICAL GUIDE # Bottom Bracket Explained: Which One Does Your Bike Have? 12 min read Nov 04, 2026 Team Zinn Cycles Nov 4, 2026, 1:24:46 AM [Team Zinn Cycles](https://blog.zinn.bike/author/team-zinn-cycles) Share: [facebook-f icon](http://www.facebook.com/share.php?u=https://blog.zinn.bike/bottom-bracket-explained-which-one-does-your-bike-have) [linkedin-in icon](http://www.linkedin.com/shareArticle?mini=true&url=https://blog.zinn.bike/bottom-bracket-explained-which-one-does-your-bike-have) [Twitter icon](https://twitter.com/intent/tweet?url=https://blog.zinn.bike/bottom-bracket-explained-which-one-does-your-bike-have) [pinterest-p icon](http://pinterest.com/pin/create/link/?url=https://blog.zinn.bike/bottom-bracket-explained-which-one-does-your-bike-have) [envelope icon](mailto:?body=https://blog.zinn.bike/bottom-bracket-explained-which-one-does-your-bike-have) The bottom bracket is one of those bike components you rarely think about until something goes wrong. Then you’re suddenly dealing with creaking, worn bearings, or a replacement part that doesn't fit. The confusion usually starts with the names. BSA, BB30, PF30, BB86, BB92, T47, square taper, and other terms can sound like completely different parts. They are actually different standards for connecting the frame, bottom bracket bearings, and crankset. Understanding the common bottom bracket types makes it much easier to identify what your bike uses, choose compatible parts, and understand why some designs are easier to service than others. ## What Is a Bottom Bracket? A bottom bracket is the bearing assembly inside the bicycle frame’s bottom bracket shell that lets the crankset rotate. The crank spindle passes through the bearings, connecting the two crank arms and allowing your pedaling force to turn the drivetrain. The bottom bracket therefore sits where the frame and crankset meet. That is why compatibility is important. A bottom bracket has to match the frame’s bottom bracket shell and the crank’s spindle. Knowing only one of those specifications isn’t enough to choose the right replacement. ## What Are the Different Types of Bottom Brackets? ![Different types of bicycle bottom brackets](https://blog.zinn.bike/hubfs/types-of-bottom-brackets.webp) Most modern bottom brackets fall into two main installation categories: threaded and press-fit. Older crank interface systems, such as square taper, Octalink, and ISIS, also remain relevant when identifying and replacing bottom brackets. The main difference is how the bottom bracket attaches to the frame and which crank spindle it fits. Understanding these distinctions helps you identify the right bottom bracket for your bike and avoid compatibility issues. ### Threaded Bottom Brackets Threaded bottom brackets use a frame shell with internal threads. The cups or cartridge screw directly into the frame, making installation and servicing relatively straightforward. Two common threaded standards are BSA and T47. #### BSA Bottom Brackets BSA, also known as English or British Standard Cycle threading, is one of the most widely used bottom bracket standards. A typical road BSA shell is 68mm wide, while 73mm is common for mountain bikes. The shell uses 1.37 x 24 TPI threading. The bottom bracket cups screw directly into the threaded shell. The drive-side thread is reversed, helping prevent the cup from loosening under pedaling forces. BSA’s main practical advantage is serviceability. A mechanic can remove the threaded cups, inspect the bearings, replace the bottom bracket, and reinstall the new unit without pressing bearings directly into the frame. BSA is compatible with a wide range of crank systems when paired with the correct bottom bracket. It is commonly found on steel, aluminum, and titanium bikes, including modern road, gravel, mountain, touring, and commuter bikes. #### T47 Bottom Brackets T47 combines a larger bottom bracket shell with a threaded interface. The standard uses an approximately 47mm-diameter threaded shell with M47 x 1.0 threading. Different T47 versions have different shell widths and bearing positions. The concept is to retain the serviceability of a threaded system while providing enough space for larger crank spindles. Press-fit standards enabled larger spindles and different frame designs, but manufacturing tolerances and bearing fit have caused creaking and servicing issues on some bikes. T47 reintroduces threads in a larger shell. This makes T47 useful on modern performance and custom frames that need compatibility with larger spindle systems while retaining a threaded interface. ### Press-Fit Bottom Brackets Press-fit bottom brackets use a smooth frame shell instead of threads. The bearing assembly or bearing cups are pressed into the frame. Common press-fit standards include BB30, PF30, and BB86/BB92. They differ in shell diameter, shell width, bearing size, and compatibility with crank spindles. #### BB30 Bottom Brackets BB30 was developed around a larger 30mm crank spindle and a press-fit bearing arrangement. Instead of threading cups into the frame, the bearings sit directly inside a larger-diameter bottom bracket shell. The standard uses an approximately 42mm shell bore. The larger spindle can help create a stiff and lightweight crank system. However, BB30 requires accurate frame manufacturing because the bearings fit directly into the frame shell. If the bearing fit is incorrect, unwanted movement can develop between the bearing and shell. This can contribute to noise and premature wear. BB30 is still found on some bikes, particularly older frames, although other standards have become more common. #### PF30 Bottom Brackets PF30 was developed to combine a large-diameter spindle with a press-fit frame interface while using cups or adapters around the bearings. The frame uses an approximately 46mm bore, with common shell widths of 68mm for road applications and 73mm for mountain bikes. The main difference between PF30 and BB30 lies in how the bearings are installed. - BB30: Bearings press directly into the frame. - PF30: Bearings are housed in cups that press into the frame. This distinction affects installation, servicing, and how the bearing assembly interfaces with the frame. #### BB86 and BB92 Bottom Brackets BB86 and BB92 belong to the PF41 family of standards. They use a nominal 41mm internal shell diameter, with the numbers referring to the approximate shell width. BB86 is commonly associated with road frames, while BB92 is commonly used on mountain bikes. These systems place the bearings inside a wider frame shell instead of using traditional external threaded cups. The main difference between BB86 and BB92 is shell width, while the 41mm bore stays the same. Choosing the correct bottom bracket depends on the specific frame and crank combination. BB86 and BB92 are especially common on carbon frames from various manufacturers. ### Traditional Crank Interface Standards Older crank interface systems are still found on touring bikes, commuters, budget bikes, and older bicycles. These systems differ in how the crank attaches to the spindle, which affects bottom bracket compatibility. #### Square Taper Bottom Brackets Square taper is an older crank interface that remains common on touring bikes, commuters, budget bikes, and older bicycles. The crank attaches to a square-shaped spindle end using a tapered interface. There are two main versions, JIS and ISO, and you shouldn’t treat them as interchangeable. Square taper systems are relatively simple and widely available. However, you need to match the spindle length to the crank and the desired chainline. If you own an older bike, a touring bike, or a utility bicycle, you may have a square taper bottom bracket. #### Octalink Bottom Brackets Shimano Octalink uses an eight-spline spindle. It was developed as an alternative to square taper, offering a larger and stiffer spindle interface. Octalink is now much less common on modern performance bikes, but it remains relevant when servicing older bicycles. If your crank uses Octalink, you need a compatible bottom bracket, not a modern 24mm or 30mm spindle system. #### ISIS Bottom Brackets ISIS Drive uses a ten-spline spindle and was another development following square taper. Like Octalink, it was designed to provide a larger, stiffer spindle interface. ISIS is now less common on modern performance bikes, but it still matters when replacing components on older bicycles. The spline design differs from Octalink, so the two systems are not interchangeable. Identify the crank interface before choosing a replacement bottom bracket. ## How Bottom Bracket Types Differ From Crank Spindle Standards Understanding the difference between frame shell standards and crank spindle standards helps prevent compatibility mistakes. Your frame has a bottom bracket shell standard, while your crankset has a spindle standard. The bottom bracket connects the two. This means you need to check both the frame and the crankset when selecting a bottom bracket. For example, a bike might have a BSA threaded shell and a Shimano crank with a 24mm Hollowtech II spindle. It needs a BSA bottom bracket specifically designed for that crank. Another bike could use the same BSA shell but have a SRAM DUB crank. In this case, it needs a different bottom bracket, even though the frame standard remains the same. ### Common Crank Spindle Sizes and Systems | Crank spindle | Common association | | --- | --- | | 24mm | Shimano Hollowtech II | | 28.99mm | SRAM DUB | | 30mm | Various modern crank systems | | Square taper | Older, touring, commuter, and utility bikes | | Octalink | Older Shimano systems | | ISIS | Older splined crank systems | ### How to Match a Bottom Bracket to Your Frame and Crankset The bottom bracket must match both the frame shell and the crank spindle. A threaded or press-fit frame standard alone does not determine which bottom bracket you need. Consider these factors when selecting a replacement: - Frame shell standard: Identify whether your frame uses BSA, T47, BB30, PF30, BB86, BB92, or another standard. - Shell dimensions: Check the shell diameter and width, as these vary across bottom bracket standards. - Crank spindle type: Identify the crank spindle system, such as 24mm, SRAM DUB, 30mm, square taper, Octalink, or ISIS. - Compatibility: Confirm that the bottom bracket supports both the frame shell and the crank spindle. Matching these specifications ensures the bottom bracket fits the frame and works with the crankset. It also helps avoid unnecessary replacements and compatibility issues during installation. ## How Do You Know Which Bottom Bracket You Have? ![How to identify your bicycle bottom bracket](https://blog.zinn.bike/hubfs/identify-your-bottom-bracket.webp) Before ordering a replacement, identify the frame shell first. - Check if it’s Threaded or Press-Fit: Look at the bottom bracket area. A threaded system has cups that screw into the frame. A press-fit system has a smooth frame bore into which the bearing assembly or cups are pressed. - Check the Frame Specifications: Your bike manufacturer’s specifications are often the easiest way to confirm the standard. Search for the exact bike model, model year, and bottom bracket specification. This can be more reliable than trying to identify the standard visually. - Measure the Shell: If the specification is unavailable, measurements can help. For press-fit systems, shell width alone is not enough. You may also need the shell’s internal diameter. For threaded systems, you need to know the thread standard and shell width. - Check the Crank Spindle: Look at the crankset or spindle markings if they are visible. A Shimano Hollowtech II crank, SRAM DUB crank, square taper crank, or another system requires a compatible bottom bracket. If you are replacing the crankset as well, confirm compatibility before buying either component. ## Does the Bottom Bracket Affect Bike Performance? Yes, but it is easy to overstate its effect. The bottom bracket must provide a secure, properly aligned bearing interface so the crank rotates smoothly. Its design also affects spindle compatibility, bearing placement, serviceability, and frame construction. However, the bottom bracket is not a magic performance upgrade. A rider’s overall experience depends on the entire bicycle, including the frame geometry, crank length, wheels, tires, drivetrain, and riding position. For custom bikes, crank selection deserves particular attention because crank length affects your pedaling position and how your legs interact with the bike. If you are considering a custom crank length, explore [Zinn’s custom cranks](https://www.zinn.bike/custom-cranks/), which offer options designed around different rider needs. ## Which Bottom Bracket Type Is Best? No single bottom bracket standard suits every bicycle. - BSA suits riders who value a familiar threaded system and straightforward servicing. - T47 offers a large threaded shell that accommodates modern crank systems. - BB30 and PF30 offer larger shell and spindle options but require correct frame tolerances and compatibility. - BB86/BB92 provides a wider press-fit design used across many modern frames. - Square taper, Octalink, and ISIS remain relevant for older or specific crank systems. The most important question is not which standard sounds best. It is which bottom bracket matches your frame and crankset correctly. ## Get the Right Crank Setup for Your Bike from Zinn Cycles [![Custom crank setup from Zinn Cycles](https://blog.zinn.bike/hubfs/custom-crank-setup.webp)](https://www.zinn.bike/custom-cranks/) The bottom bracket is an integral part of your bike’s drivetrain. When building a custom bike, crank length, spindle type, chainline, and rider fit must work together. A custom crank setup can be especially useful when standard crank lengths do not suit your leg length or riding position. Getting those measurements right before choosing the bottom bracket can help you build a drivetrain that works as a complete system. Zinn Cycles offers custom road and gravel cranks from 180mm to 220mm, along with shorter options. These choices let you consider crank length alongside rider measurements and overall bike fit, rather than selecting a fixed component. A well-matched crank setup also needs to work with the bottom bracket, spindle, and chainline. Considering these components together can help create a drivetrain that supports the rider's position and intended use. Check out Zinn’s custom crank options to find a setup built around your measurements and riding needs. [Explore Zinn Custom Cranks](https://www.zinn.bike/custom-cranks/) ## FAQs ### What is the most common bottom bracket type? BSA threaded bottom brackets remain one of the most widely used standards. However, the right type depends on your frame’s shell and the crankset you use. ### Can I use any crankset with any bottom bracket? No. The bottom bracket must match both the frame’s bottom bracket shell and the crank’s spindle standard. Always check compatibility before purchasing replacement parts. ### Are press-fit bottom brackets better than threaded bottom brackets? Neither is universally better. Press-fit systems can accommodate different frame and spindle designs, while threaded systems are often valued for straightforward installation and servicing. ### How often should you replace a bottom bracket? There is no fixed replacement interval. Replace or service it when the bearings develop roughness, play, noise, or other signs of wear. Riding conditions and maintenance can affect service life. ### Does bottom bracket type affect crank length? Bottom bracket type does not determine crank length. Crank length is a property of the crankset. However, the bottom bracket must be compatible with the crank’s spindle and position it correctly within the frame. TE ##### Written by Team Zinn Cycles ## Related articles [View all blogs](https://blog.zinn.bike/) ### [![Bottom Bracket Explained: Which One Does Your Bike Have?](https://blog.zinn.bike/hubfs/bottom-bracket-explained.webp) Bottom Bracket Explained: Which One ... The bottom bracket is one of those bike components you rarely think about until something goes wrong. Then you’re ... Team Zinn Cycles Nov 4, 2026](https://blog.zinn.bike/bottom-bracket-explained-which-one-does-your-bike-have) ### [![Titanium vs Steel vs Carbon: Which Custom Frame Material Is Right for You?](https://blog.zinn.bike/hubfs/frame-materials-1.webp) Titanium vs Steel vs Carbon: Which ... Should your custom bike be light? Stiff? Comfortable? Nearly maintenance-free? Easy to repair years from now? 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