What size bolt? Home › Forum › Bike Forum › What size bolt? Viewing 4 posts - 1 through 4 (of 4 total) What size bolt? d45ythMember Try a bolt from somewhere else on your bike that you know the size of (bottle cage mount, rotor bolt, etc.). If it is slightly bigger than a normal stem bolt though, it probably is M6. Posted 4 years ago Yet1manMember Yes, its a measure of the OD of the threads. In reality if you measured a bolt using verniers/micrometer, it would actually come out just under its indicated size ie a 6mm/M6 with probably measure around 5.7mm. Posted 4 years ago dantsw13Member Thanks – thought so, but wanted to be sure. Posted 4 years ago dantsw13Member I’ve lost one of the bolts from my Spank Spike stem. It’s slightly bigger than a standard stem bolt. Now I can’t believe I don’t know this, but is the bolt size (ie M6) an indicator of the diameter of the outside of the threads? Posted 4 years ago Viewing 4 posts - 1 through 4 (of 4 total) The topic ‘What size bolt?’ is closed to new replies.