I'm giving serious consideration to putting some K11 forks on my K100 in order to get better suspension and, more importantly, better brakes.  In doing this I want to keep the standard K100 setup with the OEM round headlight and surrounding Tupperware.

However the K1100 lower fork tree does not have the M6 mounting holes so I'll need to add those.  I've retapped stripped threads before so am not a stranger to that process but have never made holes "from scratch."

I get that you need to use a smaller than 6mm drill bit but in poking around I see references to using between 5mm and 5.25mm. In looking at my favorite online hardware store (McMaster-Carr) they have 5.0, 5.1 and 5.2mm bits available and I'm leaning towards the 5.2.

Anybody here ever drilled/tapped holes in a fork tree? Advice? Pointers?

A 5mm drill will be great for producing a M6 tapped hole with 75% thread in aluminum.  You don't need to get a 5.1 or 5.2 drill.

Metric tap sets will have three of each tap usually.  They are designated with 0, 1 or 2 rings on the shank.
1 ring - starter tap
2 rings - chasing tap
0 rings - bottoming tap



If you're tapping through holes, use the starter tap followed by the chasing tap.  If you're tapping blind holes, use the starter tap followed by the chasing tap followed by the bottoming tap.

Be cautious drilling holes in the allow of the triple clamps they have a very high percentage of Bismuth and are as tough as old boots. My advice is to set them up in a vice on a bench drill and clamp them well and use a high quality very sharp drill.
I have seen that type of alloy destroy a drill that wasn't up to scratch.