Originally Posted by
Andrew R Stewart
2- 6mm is very close to 1/4" and that's a lot more common in drill indexes here in the US. Andy
Nit-picking: in the absence of a 6.0 mm drill, I'd try drilling it to 15/64" (5.95 mm) and test-fit the bolt, before drilling to 1/4", if the extra wiggle room isn't needed. A nominal M6 bolt will often* fit fine in a hole drilled 15/64".
* Andrew knows this but not everyone here will: I have to say "often" because it varies, depending on the tolerance on the bolt, whether the bolt is all threads or has an unthreaded shank, and even depends on how sharp your drill is and your drilling technique. Drills can make holes bigger (or smaller!) than the size they claim to be.
I'll admit the difference between 15/64" and 1/4" is hardly worth mentioning for this job. It's more of an aesthetic preference of mine, than a real engineering issue. Especially for a "mere" headlight/fender mounting hole, as opposed to a brake mount. I just like removing less metal when I can, as a general rule. You can always drill it bigger later.
Mark B