The limited and no longer available factory 18650 body is known for being a little too thin bodied. That's one reason it's not made anymore. I suspect the same is true for boring but I'll be watching to see what other responses this thread garners.
have You ever compared, say, a SF 6P body which can be bored and a Quark body?
the outside diameter of the Quark is 20 mm, in general, NOT at where the o-rings sit (where the diameter is even less).
That light is built to be small and light and it perfectly does that. (just by the way: thats also what the 18650 body does. Its "problem" lies in the incompartible sizes of the remaining part's threads, needing additional threads and by that making it too long and less "safe" in use)
Outside of the issues with wall thickness, boring a light is not cheap. It doesn't seem worth spending $30-40 on a mod for a $70 light when you could just buy something that's made for 18650s in the first place.
That would still be a problem. The thread width to be compatible with a quark are too small to work with 18650's at all. The tube that was designed for 18650's was longer and had a secondary opening that allowed for the 18mm batts but also made the light longer.
I've kinda been waiting for 4Sevens to make a EDC light for 18650. My current EDC is a Quark 123*2 'Tactical' XP-G R5, and I just love the light.
If you want something just a tad bigger that utilizes 18650, then maybe try a Fenix PD31. I have one of these and I'm tempted to make it my 'new' EDC....