Measuring bolt sizes

plug1n Posts: 204
edited November 2010 in Workshop
I feel dumb asking this after over 40 years of fettling, but....

Metric bolts (I read) are measure across the bolt diameter.

When I do this on a sample of bolts using a digital caliper, I get 7.80, 6.85, 4.86, 5.92

Should I be rounding up? i.e. 6.85 = M7 ?


  • Monty Dog
    Monty Dog Posts: 20,614
    Yes - whilst most metric screws used on bikes conform to an ISO standard which includes the pitch of the thread. Standards are typically M4x0.7, M5x0.8, M6x1.0, M8x1.25, M10x1.5 you do occasionally come against a finer pitch in certain applications. M7 and M9 are unusual - I had a pre-war Citroen which used them and getting spares was a pain.
    Make mine an Italian, with Campagnolo on the side..
  • ChrisSA
    ChrisSA Posts: 455
    ISO68-1 is your friend.
  • The dia on an external thread is always 0.1-0.15 mm less than the stated size so that it will screw into the corresponding nut or internal thread, for example if an 8mm bolt was exactly 8.00 mm it would likely be very tight in the internal thread.