A bottom bracket that looks like a simple bearing replacement can become a costly compatibility exercise once the wrong cups, press-fit unit or installation tool arrives. Knowing how to identify bottom bracket standards starts with separating the frame interface from the crank spindle and then confirming both with measurements. A label on an old bearing is useful, but it is rarely enough on its own.
For workshops, this process prevents repeat bookings and improvised adapter stacks. For riders, it protects a premium crankset, preserves correct chainline and ensures the replacement can be serviced with the tools available.
The bottom bracket shell is the frame structure that houses the bearings. Its internal diameter, width and thread type establish most of the standard. The crank spindle then determines which bearing bore or reducer arrangement is required within that shell.
This distinction matters because a 24 mm Shimano-style spindle, a 28.99 mm SRAM DUB spindle and a 30 mm spindle can all be fitted to certain shell types using different bottom brackets. Seeing a particular crank fitted to the bike does not automatically identify the frame standard.
Before ordering anything, remove the crankset and inspect the exposed bottom bracket. Clean away dirt and grease so markings, shell edges and cup profiles are visible. Photograph both sides before removal, particularly if the bike uses spacers, wave washers, dust covers or preload hardware. Their order and orientation can be critical during reassembly.
The quickest initial check is whether the frame has threads. A threaded shell will show a clearly defined spiral thread inside the frame, while a press-fit shell has a smooth bore into which bearings or bearing cups are pressed.
A conventional external threaded bottom bracket usually has aluminium or composite cups visible outside both sides of the shell. BSA, also called English threaded, is the most common threaded road, gravel and mountain bike format in Australia. Most modern BSA shells are 68 mm wide for road applications and 73 mm for mountain bike applications, although gravel, e-bike and older platforms can introduce exceptions.
Italian threaded shells are less common but remain relevant on some Italian-made road frames. They are generally 70 mm wide, and the drive-side cup uses a conventional right-hand thread. BSA differs because its drive-side cup is reverse threaded. Do not use thread direction as the only identifier unless the cups are already removed and the frame origin is known.
T47 is another threaded standard seen on premium road, gravel and custom frames. It uses a much larger shell diameter than BSA and can be configured with external or internal cups. T47 naming alone is not enough: shell width and whether the frame uses an internal or external cup arrangement must be confirmed before selecting the bottom bracket.
With a press-fit frame, the bearing or cup sits inside the shell. There may be only a narrow bearing seal or dust cover visible at each side. The shell bore must be measured accurately, because several standards look similar once installed.
Digital callipers are the practical minimum for identifying an unknown bottom bracket. Measure the bare shell after removing the existing unit where possible. Record the width from face to face, then measure the internal diameter at more than one point to check for damage or an out-of-round shell.
Do not measure across a remaining cup and assume that is the shell diameter. A press-fit cup can obscure the actual frame bore, and an adapter may have been installed to suit a previous crank. If removal is not yet possible, use the old unit's part number and frame manufacturer's specification as supporting evidence rather than relying only on a calliper reading.
Common dimensions provide a useful starting point:
Those figures identify the frame interface, not necessarily the part you need to buy. For example, a 46 mm by 86.5 mm shell points to BB386EVO, but the appropriate unit still depends on whether the crank spindle is 24 mm, 28.99 mm or 30 mm.
BBright is another frequent source of confusion. It is based around a 79 mm wide shell and may be supplied in press-fit or threaded configurations depending on the frame generation. Cervelo-specific systems such as BBRight should be confirmed against the frame model and year rather than guessed from a visual inspection.
Once the frame standard is known, measure the crank spindle where it runs through the bearings. A 24 mm spindle is common on Shimano Hollowtech II and many compatible cranksets. SRAM DUB uses a 28.99 mm spindle, while many Praxis, FSA and other performance cranksets use 30 mm spindles. SRAM's older GXP system is stepped, measuring 24 mm on the drive side and 22 mm on the non-drive side.
Spindle diameter is only one part of the crank specification. Measure spindle length if changing cranksets, and check the manufacturer's required spacer arrangement. Road and mountain bike cranks can use the same nominal spindle diameter but need different shell spacing, chainline and preload arrangements.
This is particularly relevant for gravel builds. A wide spindle crank designed around a 47 mm or 52 mm chainline may physically fit a road-width shell with the correct bottom bracket, yet deliver poor front derailleur alignment or an unsuitable chainline. Compatibility is not just whether the crank turns freely.
Many bottom brackets carry laser-etched or printed markings on the cup, bearing seal or centre sleeve. Look for terms such as BSA, T47, PF30, BB30, BB86, 24 mm, DUB or 30 mm. The brand and model number can be more valuable than a generic standard label, especially where the unit is an aftermarket conversion bottom bracket.
A unit marked “PF30 to 24 mm”, for example, describes a PF30 frame interface paired with bearings for a 24 mm crank spindle. Replacing it with a standard PF30 30 mm bottom bracket would be incorrect if the current crank is retained.
Be alert for conversion systems. Thread-together press-fit bottom brackets, reducer sleeves and one-piece adapters are regularly used to improve bearing life, reduce creaking or make a particular crank compatible with a frame. The part removed from the bike may therefore not represent the frame's original specification.
Use at least two sources of evidence before committing to a replacement: physical measurements plus the frame manufacturer's technical information, or existing part markings plus measured spindle diameter. A build sheet, invoice or previous mechanic's notes can help, but should not overrule the hardware in front of you.
Also check the installation method. Threaded BSA and T47 cups require the correct spline or notch tool, while press-fit systems need drifts sized to the cup or bearing outer diameter. Carbon shells require careful surface preparation, even press pressure and a proper bearing press. Hammering bearings into a carbon frame is not a workshop shortcut.
Bearing quality and sealing should suit the bike's use. A low-friction road bottom bracket may be appropriate for a fair-weather race bike, while a gravel or mountain bike used through wet Victorian winters often benefits more from durable seals and reliable service intervals. The lightest option is not automatically the longest-lasting one.
For uncertain builds, record the shell width, shell bore, thread type, crank model and spindle diameter before contacting a supplier or workshop. DSG PRO Imports supports technically specific component choices, but accurate identification at this stage is what turns a bottom bracket purchase into a correct, quiet and serviceable drivetrain.