Threaded vs press-fit bottom brackets: BSA, T47, PF30 and DUB explained
By Crankworthy · Published · Updated
Two facts decide your bottom bracket: what the frame shell is, and what spindle your crank has. Threaded shells (BSA, T47) take cups that screw in; press-fit shells (BB30, PF30, BB86/92) take cups or bearings pressed into a smooth bore. The bottom bracket is simply the adapter between the two, and the figures below are what you match.
The standards, with their numbers
Shell dimensions and thread specs as Park Tool defines them [1]. Measure your shell before buying anything — the name on the frame spec is not always right.
- BSA / English (threaded)
- 1.37 in × 24 tpi thread, roughly 34.8 mm across the thread and about 33.7 mm shell inside diameter. Drive-side cup is left-hand threaded and tightens anticlockwise [1].
- Italian (threaded)
- 36 mm × 24 tpi, 35 mm inside diameter. Right-hand thread both sides, which is why it is more prone to loosening [1].
- T47 (threaded)
- M47 × 1.0 thread in a shell of about 46 mm inside diameter — a press-fit-sized shell with threads cut into it, so it takes a 30 mm spindle without the creak [1].
- BB30 (press-fit)
- 42 mm nominal bore, bearings pressed straight into the frame. Spindles 28.99 mm, 24 mm or 22/24 mm depending on the bottom bracket used [1].
- PF30 (press-fit)
- About 46 mm inside diameter, shells 68–86 mm wide. Bearings sit in cups pressed into the frame rather than in the frame itself [1].
- BB86 / BB92 (press-fit)
- Nominal 41 mm bore, with shell widths across the family running from 86 to 132 mm — the number is the width [1].
- Shimano Hollowtech II spindle
- 24 mm. The most widely supported spindle across all of the above.
- SRAM DUB spindle
- 28.99 mm — one spindle intended to cover shells that previously needed either a 30 mm or a 24 mm setup [2].
Why the drive-side cup is reverse threaded
On BSA and T47 the drive-side cup has a left-hand thread, so it tightens anticlockwise 1. That is not a quirk — it is what stops the cup unscrewing itself as you ride.
The usual explanation is precession: a cup sitting in a slightly larger bore, under a load that rotates around it, tends to walk around the bore in the opposite direction to the rotation. With a conventional right-hand thread that walk would slowly back the cup out. Reversing the thread turns the same effect into something that keeps the cup snug.
Italian shells use right-hand threads on both sides, which is the practical reason they have a reputation for working loose. It is also why the first thing to check on a creaking threaded bottom bracket is simply whether the cups are still tight.
What press-fit buys, and what it costs
A press-fit shell has no threads, so the bore can be wider for the same frame material — which is what frame designers wanted. The cost is that the frame itself has to hold the bearings still.
With a threaded shell, the cups clamp into threads and the bearings sit in metal. With press-fit, the interference fit between cup and frame is the only thing holding the assembly, so the bore has to be round, parallel and the right size to a tight tolerance. Carbon shells that shift slightly during moulding are where the trouble starts.
When it goes wrong it announces itself as a creak under hard pedalling, and the fix is not usually new bearings — see our guide to silencing a press-fit or BB30 creak. Thread-together bottom brackets are the structural answer: two cups that screw into each other inside the shell, so the frame no longer has to grip each bearing separately 3.
Identifying what you have
- 1
Look at how it is held in
Cups with tool notches that unscrew mean threaded. Cups or bearings sitting flush in a smooth bore mean press-fit.
- 2
Measure the shell width
With the bottom bracket out, measure across the shell. 68 and 73 mm are the threaded norms; 86.5 and 92 mm the common press-fit ones.
- 3
Measure the bore
For press-fit this is what names the standard: roughly 41 mm is BB86/92, 42 mm is BB30, 46 mm is PF30 1.
- 4
Identify the crank spindle
24 mm Shimano, 28.99 mm SRAM DUB, 30 mm, or a square taper. The bottom bracket has to match both the shell and this.
Bottom brackets in the catalog
All threaded BSA at present — match the spindle to your crank.
- Shimano BB-MT501 (BSA)Threaded BSA 68/73mm external bearing BB, 24mm Hollowtech II, alloy cups
- SRAM DUB BSA Bottom BracketThreaded BSA 68/73mm BB for DUB 28.99mm spindle cranks$37.82Amazon
- Wheels Manufacturing BSA Bottom Bracket, Shimano MTB (24mm)Threaded BSA 68/73mm BB, ABEC-3 bearings, for 24mm Shimano Hollowtech II cranks
- Shimano BB-UN300 (Square Taper)Square taper cartridge BB, BSA threaded, 68mm, common spindle lengths$18.00Amazon
What this job needs that we don’t stock
Named so you can source them elsewhere — we would rather say this than point you at a product that is not the right one for the job.
- Press-fit bottom brackets of any standard — BB30, PF30, BB86/92 — and the tools to remove and press them.
- T47 bottom brackets, and thread-together units for press-fit shells.
References
Related
Part of the bike upgrade and compatibility guide.