What a Bottom Bracket Actually Does
The bottom bracket is the bearing assembly that your crankset spins on. It's a simple job, but it's the most loaded bearing on the whole bike, taking your full body weight on every pedal stroke, thousands of times an hour, in all weathers. When it goes wrong, you feel it as creaking, clicking, roughness or play at the cranks, and if you ignore it for long enough, it can damage the frame itself.
Bottom brackets are also the most confusing part of bike specification for most riders, because there are about a dozen different standards, none cross-compatible, and frame manufacturers keep changing them. The same bike brand can use three different BB standards across three model years.
I'm going to walk you through what every standard is, why it exists, how to identify what's on your bike, and when it needs replacing.
The Main Bottom Bracket Standards, and Why They Exist
Before we go standard by standard, it helps to understand why there are so many. The BB sits inside the frame, and as frame design has changed (lighter, stiffer, wider tube diameters, different materials), the housing has had to change too. Bearings either got bigger for stiffness, smaller for weight, or moved around for frame clearance reasons. Every time the frame changed, the BB standard changed with it.
BSA (also called English Threaded)
BSA, or British Standard Cycle, is the granddaddy of BB standards. It's threaded into the frame with cups that screw in from each side, the bearings sit inside the cups, and the axle of the crank passes through the middle. It's been around in various forms since the 1950s, which is why every steel, titanium and aluminium frame made in the last 30 years has used it as the default.
Standard shell width is 68mm for road bikes and 73mm for mountain bikes, with a 1.37" x 24 TPI thread. Almost every external bearing crankset (Shimano Hollowtech II, SRAM DUB, Campagnolo Ultra-Torque excepted) uses BSA, and it has the huge advantage of being easy to install with basic tools. The downside is that the threaded interface can creak if the cups aren't torqued up properly or if the threads get contaminated.
At the SW17 workshop, BSA is still the BB I fit most often. It's the standard I recommend if you're building a steel or titanium frame, and the easiest one for home mechanics to service.
BB86 and BB90
BB86 and BB90 are Shimano's press-fit standards for carbon road frames. The bearings sit directly in the frame, pressed into moulded cups in the carbon shell, with no metal housing. BB86 has an 86.5mm wide shell with 41mm diameter bearings; BB90 is 89.5mm wide with 37mm bearings. The narrower BB86 is more common on endurance and aero road bikes, while BB90 was used on some of the older Shimano-sponsored race frames.
The advantage of press-fit is that frame designers can make the down tube and chainstays wider (no metal cup taking up space), which makes the frame stiffer. The disadvantage is that the bearings sit in plastic mouldings in the carbon, and if water gets in and the bearings corrode, they can deform the frame shell. They also tend to creak more than threaded BBs, which is why most aftermarket BB86 units now use expanding collet designs that lock the cups to the frame.
BB30 and PF30
BB30 was created by Cannondale in the early 2000s for their carbon frames. The shell is 68mm wide, the bearings are 30mm inside diameter (hence the name), and the bearings press directly into the frame with no cups. PF30 is the same idea but with a 46mm pressed-in cup that the bearings sit in.
Both standards use 30mm diameter crank axles, which is what allowed the whole bike industry to move to larger diameter spindles for stiffness. The problem with early BB30 was that the bearings were undersized for the loads, leading to creaking and premature wear. Modern PF30 with proper installation is much better, but it still needs the right tools and a good torque procedure to avoid the creak.
T47
T47 is the relative newcomer, designed to be the best of both worlds: the large diameter bearings of press-fit standards, with the threaded interface of BSA. The shell is 68mm (road) or 73mm (MTB) wide, with a 47mm x 1mm thread, and the bearings sit in cups that thread in from each side.
T47 has become the default standard on most boutique steel, titanium and carbon frames built in the last few years. It's stiffer than BSA, easier to install than press-fit, and uses standard 24mm or 30mm spindles. If you're speccing a custom build, T47 is the standard I'd recommend 90% of the time.
BBright
BBright is Cervélo's standard, used on most of their road and gravel frames. The shell is 79mm wide (asymmetric, pushed 11mm to the drive side), which lets the down tube be oversized for stiffness while still using standard 24mm or 30mm crank spindles. The bearings are 6806 size, pressed directly into the frame or into a plastic cup. It's a good standard but proprietary to Cervélo, so aftermarket BB options are more limited than for BSA or T47.
Italian Threaded (ITA)
Italian threaded is the oldest threaded standard, used on classic Italian steel frames and a few high-end titanium and carbon bikes. The shell is 70mm wide with a slightly larger 36mm x 24 TPI thread, right-hand thread on both sides. The bearings sit directly in the cups with no plastic retainer. You'll mostly come across this on vintage bikes and brand new steel builds from Italian framebuilders. BB options are more limited and more expensive, but still readily available from Campagnolo and Wheels Manufacturing.
Compatibility: Shell Width, Axle Type and the Spindle
Three things have to line up for a bottom bracket to work: the shell width of your frame, the diameter of the spindle on your crank, and the bearing size that fits both.
Shell width is fixed by the frame. Road frames are usually 68mm, 86.5mm (BB86) or 79mm (BBright). MTB frames are usually 73mm. Gravel frames can be any of these. T47 frames are 68mm or 73mm.
The spindle diameter is fixed by the crank. Shimano Hollowtech II, Race Face and most others use 24mm spindles. SRAM DUB uses 28.99mm (marketed as 29mm). SRAM's older BB30 cranks and many aftermarket cranks use 30mm spindles. Campagnolo Ultra-Torque uses a split spindle that joins in the BB, while Power-Torque uses an integrated axle. The bearing size is determined by the shell width and the spindle, and you don't need to know the bearing number, you just need to know the standard and let the BB manufacturer do the rest.
How to Measure and Identify Your Bottom Bracket
If you don't know what's on your bike, look at the frame where the down tube meets the chainstays. You'll see either a round hole with a metal cup threaded into it (BSA, T47, Italian) or a smooth round hole with no visible threads (BB86, BB30, PF30, BBright). Measure the shell width (typically 68, 73, 86.5 or 89.5mm). Then check your crank arm for a brand and model number.
If the frame has threaded cups, count the thread direction. Left-hand thread on the drive side means BSA or T47. Right-hand thread on the drive side means Italian. If there's no visible thread and the bearings sit in a smooth bore, it's a press-fit standard. Most modern cranks will have a sticker or laser-etched model number on the back of the crank arm. Note that down, look up the BB standard on the manufacturer's website, and you'll have your answer in five minutes.
When to Replace a Bottom Bracket
There are four clear signs that a BB needs replacing.
Roughness under load. Spin the cranks with the chain off. They should turn smoothly and silently. If you can feel or hear the bearings, they need replacing.
Play in the cranks. Hold the crank arm and try to rock it towards and away from the frame. Any movement at all means the bearings are worn or the BB cups have come loose. On a threaded BB, the cups may just need re-torquing. On a press-fit, the bearings have probably had it.
Creaking that you can localise to the BB area. This is the classic press-fit BB problem, and it's usually caused by dried-out or contaminated bearings, or cups that have moved in the frame. It can also be a loose chainring or pedal, so rule those out first.
Visible water ingress or rust on the spindle or bearings. If the bike has been pressure-washed, ridden through deep puddles, or stored outside, water will eventually get in. In terms of mileage, a good quality BB in a UK climate will typically last 3,000 to 5,000 miles for a budget Shimano unit, and 6,000 to 10,000 miles for a premium unit like Chris King or Hope. The cheap ones go faster, but you can usually hear them before they fail completely.
Cost and Time for BB Replacement at Our Workshop
A bottom bracket replacement at BikeClinique takes about 30 to 60 minutes for a threaded standard, or 45 to 90 minutes for a press-fit. Labour is £40 to £75 depending on the standard and the frame. The BB itself ranges from £25 to £50 for a Shimano or SRAM unit, up to £150 to £300 for a Chris King, Hope or Ceramic Speed. Parts are always quoted separately so you can choose what you want to fit.
If you're having a groupset upgrade done at the same time, the BB replacement is included. Read more on our Bike Groupset Upgrade page. If the BB shell in the frame is damaged (which happens with aluminium press-fit frames that have had corroded BBs in them for years), the frame usually can't be repaired, and we'll tell you honestly before starting any work.
Common London Workshop Scenarios
A few typical situations I see in the workshop.
Wet commuter with a press-fit BB. The bike has been ridden through a London winter for two or three years, the BB is crunchy, and there's visible rust on the axle. The fix is a new BB with a proper fitting that includes threadlocker and anti-seize on the cups. Labour around £60 plus parts.
Vintage steel road bike with Italian threading. Often the BB has never been replaced and the bearings have basically welded themselves into the cups. We have to soak the cups in penetrating oil, sometimes overnight, before they'll come out. The new BB is more expensive (Italian-threaded units start around £50) but the install is straightforward once the old one's out.
Carbon road frame with BB86 that's creaking. The first fix is usually to remove the BB, clean the shell thoroughly, apply carbon assembly compound, and refit with a fresh unit. This resolves 80% of BB86 creaks without needing new parts.
Custom build on a new steel or titanium frame. Almost always specced as T47 these days, and almost always a joy to work on. The wider shell and threaded interface make it stiff, reliable, and creak-free if installed correctly. If you're building up a new frame or have a BB problem on an existing one, the workshop in Tooting can handle every standard. Get in touch via the contact page for a quote, and we'll get it sorted.
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