PSA: T47 Internal BB width is now 85.5mm

After some head-scratching with Nick @Neuhaus_Metalworks and an Ekar Crankset, we came to the realization that the industry had secretly adopted 85.5mm as their T47 Internal Bearing (IB) BB width.

  • The standard was 86.5mm
  • Paragon BB shells arrive at 87.5mm to give you some wiggle room to square up the distortion
  • After clean-up, the BB shells are typically 87mm wide
  • Turns out the standard is now 85.5mm

This is how I think it went down:

2015 T47 is born at 86.5mm

T47 is introduced with the help of Argonaut, Chris King, and PMW:

The idea was to take the PF30 shell and make it threaded. Technically this was to allow you to thread PF30 BB’s after building. but I have only seen someone do this once.

T47 adopted both the diameter and width of the PF30, which is 86.5mm

~2019 Trek enters T47 at 85.5mm

Trek was the first major bike brand to adopt T47 on one condition: they changed the bb width to 85.5mm to allow more tool purchase on the BB splines. Their argument was that for mass production, they needed an extra .5mm of flange width on the BB. They partnered with Praxxis for their OEM BB’s.

They also argued that having a narrower BB shell was not an issue for their frames. It would be compatible with all existing BB’s because most cranks can take up 1mm of spindle length or at worst, you use a spacer.

Source: article on Cyclingtips (no longer exists).

2019-2020 Drivetrain companies adopt 85.5mm

Campy and SRAM are the only drivetrain manufacturers that officially adopted the T47 standard. Both use 85.5mm for their T47 nominal width and 86.5 for the PF30 width. Since SRAM works closely with Trek, I’m sure this was the plan all along.

So there you have it, T47IB is officially 85.5mm wide. So give your T47 facing tool a few extra turns and let your parts suppliers know!


Does Neuhaus/other builders use 87.5mm shells and face off the extra 2mm supplied or are you getting specific 85.5mm T47 shells from another supplier? This seems like a lot of material for the facing tool to remove, maybe machining the width in a lathe before welding could be a better approach?

Paragon now makes an 86.5mm shell in Titanium. Not sure what their plan is for steel.

NMW uses 73mm T47 for steel mtbs and BSA for steel gravel/road, so we have not come across the 85.5mm issue with steel bikes.

Using a nominal 86.5mm wide shell should still work with all drivetrains, EXCEPT Campagnolo.

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I will be interested to hear from Paragon on this but early T47 was available in 68mm and 73mm width. The announcement doesn’t mention a width that I could find in my fast scan of that article. I remember reading it back in 2015. I have a couple out there from 2015-16 with Phil square taper BBs in 68mm and a few 73mm wide ones in my box that will likely never get used! I have standardized on 86.5 for the past couple years.

I think the T47 external bearings (EB) 68mm and 73mm will continue to exist. The external bearing is not a very common standard for two reasons:

  • Road bikes get more internal routing space with 85.5 T47IB (internal bearing). I think 85.5IB makes more sense than 68EB
  • metal mountain bikes are not popular, except in the entry-level market, where BSA will be the cheapest.

That being said, I think the EB standard will stick around. There are few bikes that use this standard, like the Ibis Haka. The Cevelo R5CX and Factor Ostro VAM use a hybrid internal and external bearing bottom bracket:


Any turn around for the Campy?

I got the Ekar as well, and was thinking to move from the T47 68mm for the next project however PMW only have the 86.5mm.
Will the extra facing cause any issue besides the early worn out on the reamer?

These are the downsides I can think of for T47 68mm:

Paragon has a great team. It is worth asking them if someone can trim an 87.5 BB shell for you manually before they ship it.

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