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I need to place several threaded holes in some large aluminum heatsinks. These are needed to mount some power resistors with 1/8" mounting holes. I've looked at several charts to try to determine which size of bit to use prior to thread-tapping them but can't quite grasp the terminology. "Just a bit* smaller" doesn't seem quite scientific enough, though.
This chart shows what appears to my mind as inverse numbers, which make it look like I'm to drill a larger hole than the thread diameter.
Screw size-4, major diameter-.112, clearance hole-.125
Thanks for any replies. 
* pun intended
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