Metric Hex vs Standard Hex compatibility.

It’s rare these days, unless working on bicycles from mid-20th century and older, to encounter any bolt/nut that isn’t metric. On the contrary, it’s somewhat common to see the head of a metric hex bolt that has been rounded due to a standard sized hex wrench being improperly applied.

Most hex sizes found on your bike will be in the 3mm-8mm range. On occasion smaller sizes are used(shifter covers), and sometimes larger(freehub body).
It is crucial to use the proper metric hex wrench on your bicycle as to prevent your bolt heads from rounding out.

There are two exceptions, at least within the 3mm-8mm range we discussed, that won’t damage your bolts in case you only have a standard hex set.
4mm is practically the same exact size as 5/32″.
8mm 
is practically the same exact size as 5/16″.

Please take caution when using standard hex wrenches on metric bolts. If you feel as if the hex wrench you are using is slightly too small, stop wrenching immediately and determine which wrench is appropriate for your specific application.

One thought on “Metric Hex vs Standard Hex compatibility.

  1. My bike’s a 1987 English light roadster, and the quirkiness off it is all the bolts are metric but only half the threads are, the rest are either imperial or Raleigh proprietary 26tpi. There’s something ridiculous about owning a BSA bicycle that doesn’t use stand BSA fittings…

    Liked by 1 person

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