Bottom Brackets
Cycling · 10 specs
Direct Answer
Bicycle bottom bracket (BB) shells connect the frame to the rotating crankset spindle. Traditional standards include BSA English Threaded (68/73 mm shell width, 1.37" × 24 TPI, reverse left-hand thread on drive side) and Italian Threaded (70 mm shell, 36 mm × 24 TPI right-hand both sides). Modern press-fit and oversized standards include BB30 (68/73 mm × 42 mm press-fit), PF30 (68/73 mm × 46 mm), BB86/92 (86.5/91.5 mm × 41 mm), and T47 (threaded M47 × 1.0 mm).
Source: ISO 6698 (Bicycle bottom bracket threads) / T47 Open Standard Specification · Last reviewed Aug 28, 2026
Specs
| BSA English Threaded Shell Width | 68.0 (Road) / 73.0 (MTB) mm shell width |
| BSA Thread Specification | 1.37 in × 24 TPI (ISO 6698) Left-hand drive side / Right-hand non-drive |
| Italian Threaded Shell | 70.0 mm width / 36 mm × 24 TPI Right-hand thread on both sides |
| T47 Threaded Standard | 68.0 / 73.0 / 86.5 mm width M47 × 1.0 mm internal thread (fits 30 mm & DUB) |
| BB30 Press-Fit Standard | 68.0 (Road) / 73.0 (MTB) × 42.0 mm (42 mm bore, bearings pressed directly into shell) |
| PF30 Press-Fit Standard | 68.0 (Road) / 73.0 (MTB) × 46.0 mm (46 mm bore with composite bearing cups) |
| BB86 (Road) / BB92 (MTB) Press-Fit | 86.5 (Road) / 91.5 (MTB) × 41.0 mm (41 mm bore, designed for 24 mm spindles) |
| Shimano Hollowtech II Spindle Diameter | 24.0 mm chromoly steel spindle |
| SRAM DUB Spindle Diameter | 28.99 mm aluminum alloy universal spindle |
| BB30 / 30mm Spindle Diameter | 30.0 mm oversized alloy spindle |
Threaded vs. Press-Fit Evolution & The T47 Revolution
- The BSA English Threading Standard: BSA (1.37" × 24 TPI) is widely regarded as the most durable and reliable threaded bottom bracket standard, avoiding the creaking issues common to press-fit designs. The right-hand (drive) side uses reverse left-hand threads to counteract mechanical precession, which would otherwise loosen clockwise threads during forward pedaling.
- The T47 Open Standard: Developed by Chris King and Argonaut Cycles in 2015, T47 pairs the wide 47 mm shell diameter of PF30 (allowing oversized 30 mm and 28.99 mm DUB spindles) with metal threads (M47 × 1.0 mm) to completely eliminate press-fit creaking and frame ovalization.