Bottom bracket standards confuse even experienced riders, and for good reason: there are more than a dozen in circulation, and picking the wrong one means a part that simply won’t fit your frame. This guide explains what a bottom bracket standard actually is, walks through the threaded and press-fit families you’ll encounter, and shows you how to identify exactly which one your bike uses before you buy a replacement.
What Is a Bottom Bracket?
The bottom bracket is the set of bearings that sits inside the frame’s bottom bracket shell and allows your crankset to spin freely. Every pedal stroke you make passes through it, so despite being one of the least glamorous parts on a bike, it is doing an enormous amount of work. A bottom bracket “standard” simply describes the combination of shell dimensions, interface type (threaded or press-fit), and bearing arrangement that a given frame and crankset are designed around.
The reason standards matter is compatibility. A bottom bracket must match three things at once: the internal or external diameter of the shell, the width of the shell, and the diameter of the crank axle (spindle) that passes through it. Get any one of these wrong and the part will not install correctly. Understanding these three measurements is the key to cutting through the jargon.
Why So Many Bottom Bracket Standards Exist
Bottom bracket proliferation is largely a story of frame builders chasing stiffness and weight savings. Traditional threaded shells are narrow, which limits how large the bearings and axle can be. When carbon frames arrived, manufacturers realised they could mould wider shells with larger bearing seats directly into the frame, allowing for oversized axles that are stiffer and lighter. Each brand pursued its own version of this idea, and the result was an alphabet soup of competing formats.
Broadly, every standard falls into one of two camps: threaded, where the cups screw into a threaded metal shell, and press-fit, where the bearings or cups are pressed into a bare shell with an interference fit. Almost everything else is a variation on shell width and axle diameter within those two families.
Threaded Bottom Bracket Standards
BSA (English) Threaded
BSA, also called English threaded, is the most common and arguably the most trouble-free standard. The shell is 68mm wide on road bikes and 73mm on mountain bikes, with a 1.37 inch x 24 TPI thread. Crucially, BSA uses a reverse (left-hand) thread on the drive side, which means the natural rotation of the cranks actually tightens the cup rather than loosening it. If you value a creak-free, easily serviced bike, a BSA frame is the safe choice.
Italian Threaded
Italian threaded shells are 70mm wide with a 36mm x 24 TPI thread, and both cups use a standard right-hand thread. That right-hand drive-side thread is the format’s weakness: it can loosen under pedalling load, so Italian bottom brackets need proper thread-locker and torque. You’ll mostly find this standard on older or boutique Italian frames.
T47 Threaded
T47 is the modern compromise. It combines the large 46mm internal diameter of a press-fit shell with reliable threads, giving frame designers room for oversized axles while keeping the serviceability and quiet running of a threaded interface. It comes in internal-bearing and external-bearing versions and has become popular on premium gravel and endurance bikes.
Press-Fit Bottom Bracket Standards
BB86 / BB92 (Shimano Press-Fit)
BB86 (road, 86.5mm shell) and BB92 (mountain, 91.5mm/92mm shell) press bearings held in nylon or composite cups into a 41mm shell. They are designed around a 24mm steel Shimano-style axle, which makes them widely cross-compatible with common cranksets. When installed with care and the correct press tool, they run well; problems usually trace back to poor installation rather than the standard itself.
BB30 and PF30
BB30 uses a 42mm shell into which the bearings press directly, paired with an oversized 30mm aluminium axle for extra stiffness. PF30 takes the same 30mm axle concept but presses the bearings inside composite cups into a wider 46mm shell. Both are stiff and light, but the direct-press interfaces are the ones most associated with the notorious press-fit creak if tolerances or installation are less than perfect.
BBRight, BB386EVO and DUB
These are wider variations designed to combine a big shell with a big axle. BBRight and BB386EVO widen the shell to fit a 30mm axle with more bearing spacing for stiffness. SRAM’s DUB uses a 28.99mm axle that splits the difference between 24mm and 30mm systems, and is offered in both threaded and press-fit shell formats. The takeaway is that shell width and axle diameter are the two numbers that actually define each of these formats.
How to Identify Your Bottom Bracket Standard
You can usually identify your standard with a caliper and a few minutes. Work through these steps:
- Look for threads. Remove the crankset and shine a light into the shell. Visible metal threads mean a threaded standard (BSA, Italian or T47). A smooth bore means press-fit.
- Measure the shell width. Use a caliper across the shell. 68mm or 73mm points to BSA; 70mm to Italian; 86.5mm to BB86; 90-92mm to BB92.
- Measure the internal diameter. A threaded shell around 34-36mm is BSA or Italian; a 46mm bore is T47 or PF30; a 41mm bore is BB86/92; a 42mm bore is BB30.
- Check the axle diameter. Measure the crank spindle: 24mm is Shimano-style, 30mm is BB30/PF30, and roughly 29mm is SRAM DUB.
If you are unsure, note the exact frame model and search the manufacturer’s specification, which always lists the bottom bracket standard. This is also the moment to double-check your crank spindle, because the axle diameter dictates which bottom brackets will physically fit.
Threaded vs Press-Fit: The Creak Debate
The long-running argument between threaded and press-fit comes down to tolerances. Threaded shells locate the bearings against precise machined threads, so they are forgiving and easy to service at home with a single external bearing tool. Press-fit relies on the frame’s bore being manufactured to tight tolerances; when it is, press-fit is light, stiff and silent, but a bore that is fractionally out of spec can allow the cups to shift under load and produce that maddening creak.
In practice, a well-made press-fit frame installed with the right press and a little assembly compound will be trouble-free, while a neglected threaded bottom bracket can still creak if the cups are dry or under-torqued. The standard matters less than the quality of installation. This is closely tied to your drivetrain setup overall, so it is worth understanding cross-chaining and gear selection alongside your bottom bracket choice.
How to Choose a Compatible Bottom Bracket
To buy the correct bottom bracket, match three specifications in order: the shell standard (which you identified above), the crank axle diameter, and the shell width. Manufacturers such as Shimano, SRAM and third parties like Wheels Manufacturing publish compatibility charts that map crankset to frame standard, and these are the fastest route to a guaranteed fit.
Keep in mind that some cross-compatibility exists thanks to adaptor bottom brackets, which let you run, for example, a 24mm Shimano crank in a PF30 frame. These adaptors are a legitimate fix, though they add a little complexity. Your crank arm length is a separate decision from the bottom bracket, and if you are changing cranks it is worth reading up on crank length at the same time. Bottom bracket choice also interacts with Q factor, the distance between your pedals, which affects fit and comfort.
Maintenance and When to Replace
A bottom bracket is a wear item. Spin the cranks with the chain off: if you feel roughness, grinding or notchy resistance, or if there is lateral play when you rock the crank arms side to side, the bearings are worn and it is time to replace. Water and grit are the main enemies, so riders in wet conditions will get through bottom brackets faster.
Replacing a threaded bottom bracket is a straightforward home job with the correct tool and a torque wrench, typically 35-50Nm depending on the cups. Press-fit replacement needs a proper press and a bearing extraction tool, and if you are not confident it is worth having a shop do it to avoid damaging the frame. Whichever standard you run, a light smear of grease or assembly compound on the interface and correct torque will prevent most creaks. If you are comfortable working on your own gears, it pairs naturally with learning how your bike’s gears work.
Frequently Asked Questions
Can I convert my frame from press-fit to threaded?
Not directly, because the shell dimensions are moulded or machined into the frame. However, thread-together (or “thread-in”) press-fit bottom brackets exist for many press-fit shells. These use two cups that thread into each other inside the shell, capturing the frame between them. They dramatically reduce creaking and are the single best upgrade for an unhappy press-fit bike, without altering the frame itself.
How long does a bottom bracket last?
There is no fixed mileage, but most riders get several thousand miles from a quality bottom bracket. Sealed cartridge units in dry conditions can last years, while riding through winter grit and pressure-washing after every ride will shorten that life considerably. Check for play and roughness a couple of times a season and replace as soon as the bearings feel notchy.
Do I need a specific tool for my standard?
Yes. External-bearing threaded cups use a splined wrench that matches the cup, T47 needs a large external tool, and press-fit installation requires a bearing press with the correct guides. A bearing extraction tool is also needed to remove worn press-fit units. Buying the right tool for your standard is far cheaper than repairing a frame damaged by improvising.
Final Thoughts
Bottom bracket standards look intimidating, but they reduce to two families and three measurements. Identify whether your shell is threaded or press-fit, measure the shell width and internal diameter, and check your crank axle diameter, and you will always be able to find a compatible part. Threaded standards like BSA and T47 remain the most fuss-free, while modern press-fit formats deliver stiffness and low weight when installed properly. Once you understand the three numbers that define each standard, the alphabet soup stops being a mystery.
