From BSA to T47: Demystifying the Most Common Bottom Bracket Standards
how to choose a Bottom Bracket for your bike?
Bottom brackets are small parts, but they carry a big responsibility. They bridge the crankset to the bicycle frame, transferring power from your pedals to the wheels while allowing smooth rotation. Over the years, bicycle designers have used a variety of shell sizes, thread forms, and bearing arrangements. That variety can feel like a maze to new riders and even to seasoned cyclists upgrading components. This article cuts through the confusion, explaining the most common bottom bracket standards in plain terms and giving you a practical guide to choosing the right one for your bike.
Understanding the basics: what a bottom bracket does and why standards matter
At its core, a bottom bracket consists of a shell that is threaded or smooth inside, bearings, and cups or adapters that hold the bearings and interface with the crank spindle. The “standard” is essentially the size of the shell (its diameter and width), the presence or absence of threads, and the pitch and diameter of those threads. The standard determines which cups, bearings, and cranks will fit together without binding or slippage. When you buy a replacement or upgrade, you need a compatible combination of shell, spindle diameter, and bearing type.
Threaded vs. press-fit: two broad families
– Threaded bottom brackets: In these systems, cups thread into the frame’s shell, and bearings sit inside those cups. The threaded approach is durable, serviceable, and widely supported across a range of cranks. The most familiar example is the BSA/English threaded standard (often referred to as “threaded” or “threaded bsa bottom bracket” in product listings). With threaded systems, you can usually service or replace bearings without removing the crankset, and you can often swap out cups to adjust for different bearings or seals.
– Press-fit bottom brackets: In contrast, press-fit systems push (press) bearings directly into the frame’s shell, with no external threads holding the cups in place. These can save weight and, in some designs, improve stiffness. However, they require precise frame tolerances and may demand special tools for installation and maintenance. Common examples include PF30 and BB86/BB92. If your frame is press-fit, you’ll typically need adapters or specific crank/bearing combinations designed for that shell.
The most common threaded standards you’re likely to encounter
– BSA/English threaded (the classic standard): The familiar 1.37-inch diameter cup threads at 24 threads per inch (24 TPI). Shell width is usually 68 mm for road bikes and 73 mm for many mountain bikes, though other widths exist for specialized frames. This is by far the most widely supported standard, which translates into broad compatibility with a huge range of cranks, adapters, and serviceable bearings.
– Italian threaded: A 36 mm diameter with 24 TPI, typically using a 70 mm shell width. Italian threading is robust and popular on some European frames, especially older Italian builds. It’s less common on new frame designs than BSA but important to recognize for retrofits or second-hand frames.
– French threaded: An older standard with distinct dimensions (historically a 35 mm shell and specific thread pitches). You’ll encounter it less often today, mostly on vintage frames. If you’re dealing with a French-thread frame, you’ll need a specifically matched bottom bracket, or you may be better off upgrading the frame or choosing a modern conversion approach.
– Swiss, other historical threads: There are a few other regional threaded standards that appear on older bikes. In practice, most new bikes will either be BSA or Italian threaded, with the occasional French frame in certain niches.
A newer and increasingly popular approach: T47
– T47 is a larger, threaded shell standard designed to combine the durability of a threaded interface with the broader tolerances and performance benefits of modern bearings. The skeleton of T47 is a 47 mm inner diameter shell with threads, allowing the use of standard threaded cups and a wide array of bearing choices. This system has gained traction in frames that seek a more modern, robust bottom bracket interface while preserving tool-friendly serviceability.
– Why builders and riders like T47: It supports contemporary crank architectures and can accommodate a broad range of spindle diameters (including 24 mm and 30 mm systems) through compatible cups and adapters. It’s particularly appealing to frames built for versatility, retrofit potential, and long-term serviceability.
A quick guide to matching cranks and shells
– Know your frame shell: If you have a newer road or mountain frame, check the manufacturer’s spec sheet or stamped markings on the frame to confirm the shell standard (BSA, Italian, French, T47, etc.). Some frames provide markings or a sticker near the bottom bracket area with the standard.
– Know your crank spindle: Cranks come with different spindle diameters (24 mm is common for many road cranks, 30 mm is common for many modern performance cranks, and some older systems use 22 mm or other sizes). Your bottom bracket must accommodate the crank’s spindle diameter.
– Bearings and service life: Consider whether you want a sealed cartridge bearing (often easier maintenance and longer life) or a serviceable cup-and-cone arrangement. Sealed cartridges are common in threaded BSA systems and many T47 configurations, but always confirm the exact bearing type with your chosen cups.
– Frame width and compatibility: Pay attention to shell width (68 mm, 73 mm, 70 mm, etc.). If you buy a new bottom bracket, ensure the cups fit the frame’s width. Some frames are designed with variable width in mind and require specific units or adapters.
How to choose a bottom bracket for your bike in practice
1) Identify your frame’s standard
– Inspect the bottom bracket area for labeling, check the frame’s manual, or contact the manufacturer. If you have access to the frame’s serial number or model, you can often lookup the exact shell standard.
– If you’re working with a used frame, measure the shell diameter and width, then compare with known standards. A reputable bike shop can help you read the shell and confirm compatibility.
2) Identify your crankset
– Determine the spindle diameter and the mounting interface your crank requires (for example, Hollowtech II, GXP, or other systems). The crank’s documentation will specify what bottom bracket