What is a T47 Bottom Bracket?

What is a T47?

Many might wish it to be a new experimental jet fighter. Instead, it is yet another bottom bracket standard. Launched in 2015 as a cure for the most common complaint of the press-fit systems: the dreaded creaky bottom bracket. In this article we’ll look at some of the characteristics of the T47 system.

Think of T47 as a larger version of traditional, and well-loved BSA threaded BB shells. T47 bottom bracket shells actually start life as standard 46mm inner diameter shells and are then threaded to M47x1.0mm. Much like a BSA shell, the right, or drive side, of the shell is left hand thread, and the left side, or non-drive side, of the shell is right hand thread.

Inside Tip: If you have a customer with a creaky metal PF30 or 386EVO frame you may be able to run T47 taps through it and install a more secure T47 bottom bracket.

T47 is Secure

A potential problem with press fit systems is the interface between the frame’s bottom bracket shell and the bottom bracket cups themselves. With metal frames such as steel and aluminum, BB shell tolerances should be held accurately by using cutting tools post welding and fabrication. A reamer and facing tool should create a perfectly round and faced BB shell. However, with carbon BB shells, once the carbon is set you are at the mercy of the layup job. On a carbon frame, you can't use a bottom bracket reamer and facer to precisely set the BB shell bore diameter and trueness. The end result is a carbon BB shell that fails to hold the cups secure in the frame. BB cups that move in a frame are going to make noise.

Threaded fasteners have been around for over 2,000 years. They are a proven system of securely holding parts in place. This has been proven with traditional BSA threaded bottom brackets. Having one set of threads as a permanent part of the bottom bracket shell allows for the BB cups to be tightened directly into the frame, preventing any type of movement. No movement of the cups means no creaking.

T47 is Adaptable

Another advantage of the T47 is it’s adaptability to different crank spindle diameters and frame widths. With larger cups, T47 can accommodate bearings compatible with 30mm spindle cranks, 29mm DUB, 22/24mm GXP and 24mm Shimano crank spindles all while giving you the option of either inboard or outboard bearing configurations. Press-fit systems have evolved over the past few years to include shell inside diameters as small as 37mm. These small diameter press-fit shells are not ideally suited for today's larger diameter cranks.

Because each cup independently threads into the frame, T47 cups can be used on any width bottom bracket shell from 68mm to 132mm wide. The determining factors for which T47 bottom bracket cups to use are the width of the shell and the length of the crank spindle. For more information on how to determine the proper T47 bottom bracket for a frame, click HERE.

T47 is Stiff

The bottom bracket junction comprising the seat tube, down tube and chain stays is a key part of a frame’s stiffness. Bottom bracket shells with smaller inner diameters such as BB86 generally have to resort to wider bottom bracket shells and beefier tubes to achieve what the PF30 (46mm) starting platform can do with smaller diameter tubing. Because a T47 shell starts with a 46mm inside diameter, and has no width restrictions, frame builders have a rather large platform to build up the bottom bracket area of the frame the best way they see fit to achieve their frame's performance goals.

T47 is Here to Stay

While it has taken a few years for T47 to gain acceptance, we have seen a steady increase in small and custom builders choosing to build frames with it. Especially common on many custom titanium and steel frames, there are also a few brands using aluminum T47 shells in carbon frames (Rodeo Labs, Ibis). The most exciting development is that Trek has begun to build frames with T47 shells. The 2020 Domane frames are built around T47 shells, some of them shipping with Wheels Manufacturing BBs installed.