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?

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.

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.

Thanks – thought so, but wanted to be sure.