Bearing Locknuts

I am looking for the dimensional tolerance required on a shaft for an M25 bearing locknut.


I checked my bearing catalogs and some older metric handbooks:

  • The locknut thread ( I assume you are using standard KM5 locknut or similar) is M25x1.5 5H
    (5H is the thread tolerance). With this internal thread you can use the M25x1.5 6g thread. The “g” locates the tolerance so, that there is a small clearance.
  • I was not able to find anything about the external thread tolerance in my books, but this is how the metric thread tolerances work and I am confident that this combination is OK. It will also create small clearance for locating the bearing on the 25mm shaft (usually 25j6 or 25k6, both are larger than 25g6)
  • Just in case that you are not familiar with ISO fits: The letters in the alphabet before h/H combined with H/h tolerances create clearance. If you want interference fit, use H hole (or h shaft) with shaft k,l,m,n… (hole K,L,M,N…)

Actual numerical values for the tolerances you’ll find in the Machinery’s handbook and many bearing catalogs.

Above is a snippet.

@gearguru, Thank you for the clear and thoughtful reply.