I have a square taper bottom bracket. How can I tell whether it is a J.I.S. or ISO taper with a minimum amount of disassembly?

FWIW, it's a Ritchey from 2004. However, it would be nice to get answers that apply to other brands as well.

  • 1
    I'm a little disappointed that Sheldon doesn't give some measurements you could mike. Based on Google, only one person has ever chosen to measure the the two, and he declined to publish his numbers. (BTW, he claims that the JIS taper is 1.7 degrees vs 2 degrees for ISO.) – Daniel R Hicks Jul 4 '12 at 18:53
  • On that link you posted, it lists various manufacturers that use each type of taper. Ritchey is listed under J.I.S. The general rule of thumb is that Japanese bikes use J.I.S. and that European makes use ISO. Also, from the article you linked to, they are mostly interchangeable. – Kibbee Jul 4 '12 at 19:33
  • Also in the Sheldon article, is says that both have an angle of 2 degrees, but that ISO spindles are longer, and taper down to a smaller end. – Kibbee Jul 4 '12 at 19:52
  • @Kibbee, yeah, I saw that Ritchey was in the JIS list. I'd love some way to verify that independently. Regarding the interchangeability, it sounds like the length spec gets confusing, so I'd like to avoid that. – Reid Jul 5 '12 at 3:20
  • I know that in general it is a safer to use an ISO crank on a JIS taper, and less reliable to use a JIS crank on an ISO taper (because the longer/smaller taper end poses a risk of poking all the way through your crankarm. However, I've never done it but Sheldon says it is OK.) – WTHarper Jul 5 '12 at 15:11

The width across the narrowest part of the flats (end of spindle):

JIS = 12.65mm

ISO = 12.33mm

Use calipers.

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    Gidday and welcome to SE Bicycles. Thank you for your first answer - please keep it up. – Criggie Mar 20 '16 at 1:24

I can't find anything that states exactly what size the taper goes down to either, so you would need a known JIS/ISO to compare with. Other sources conform they are the same angle though. Minimum disassembly will be taking off one crank.




If its the length of the taper that matters, let measure that.

JIS = 16.96mm


  • This answer could benefit from a photo showing exactly where you measured from and to. – Criggie May 6 '18 at 1:40

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