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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.

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    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
    Commented Jul 4, 2012 at 18:53
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    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
    Commented Jul 4, 2012 at 19:33
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    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
    Commented Jul 4, 2012 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
    Commented Jul 5, 2012 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
    Commented Jul 5, 2012 at 15:11

5 Answers 5

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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 ♦
    Commented Mar 20, 2016 at 1:24
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The easiest method is measuring the outmost part of the spindle, like the answer of Pedalphile suggest, however there seem to be differing measurements.

These are the values I found(source in german):

ISO 1991: 12,50 mm

JIS: 12,63 mm

ISO 2015: 12,73 mm

I was able to to reproduce the ISO 1991 value by measuring my own bike.

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  • Does anyone really use the new ISO standard? As far as I know Campagnolo was the last one to use ISO and they switched to two piece system before 2015.
    – ojs
    Commented Aug 4, 2021 at 5:29
  • @ojs certain (expensive) track cranks (by japanese manufacturers!) still use ISO. it isn't clear if they use the newer standard.
    – Noise
    Commented Aug 4, 2021 at 6:40
  • @JoeK Which ones?
    – ojs
    Commented Aug 4, 2021 at 8:39
  • Sugino would be one example @ojs
    – Nate W
    Commented Aug 5, 2021 at 23:15
  • Interesting. I have Sugino Pista cranks on one bike, they were sold as JIS and fit on JIS BB as they should
    – ojs
    Commented Aug 6, 2021 at 5:29
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This is a minor answer expanding on a comment I made to the original post. If the crank in question was Shimano, then the BB has to be a JIS taper, unless someone didn't know that Shimano square taper cranks have always used the JIS taper. Similarly, a Campagnolo square taper crank would always use ISO taper.

Unfortunately, with other crank manufacturers, the picture may be less clear. For example, as ojs pointed out in comments, TA and Stronglight (two French manufacturers) probably used ISO for a long time, but I can confirm that current TA square taper cranks are designed for the JIS taper, because I have one. ojs points out that older European brands may be ISO.

The bike model and year may not offer us much information if we know the crankset and BB in question. A 2004 Ritchey road bike could conceivably have been built up with Shimano or Campagnolo cranks, or maybe even some obscure third party if the owner wanted that. If it were a mountain bike, then I'm less familiar, but I assume that would shift the potential set of components away from the smaller European brands, so I would assume that the JIS taper is more likely.

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    Still nitpicking, the question could be read that it's a Ritchey bottom bracket. Ritchey used to sell bottom brackets and cranks under their brand, and the concensus seems to be that at least the cranks were JIS.
    – ojs
    Commented Aug 6, 2021 at 6:38
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You could use a known JIS/ISO to compare to.

Other sources confirm they are the same angle though. Minimum disassembly will be taking off one crank.

http://www.urbanvelo.org/issue15/urbanvelo15_p86-87.html

http://www.velodromeshop.net/index.php?p=catalog&parent=217&pg=1

I can't find anything that states exactly what size the taper goes down to.

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ISO? It may be something like 14mm, measured at the flats; in other words the diameter of the spindle. JIS is probably measured at 16.9mm.

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    Is that correct? This contradicts a previous answer, plus it’s not 100% clear where you are measuring.
    – Weiwen Ng
    Commented Jul 25, 2022 at 1:52
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    "maybe something like" and "probably" doesn't sound like they actually measured anything
    – ojs
    Commented Jul 30, 2022 at 19:40

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