Thanks to all for help. This worked great, rear rack is back on.
For anyone else needing to do this here's what I did.
- M6 x 1 (coarse) tap
- 13/64 bit - made a couple passes after going through.
- used 3-in-one oil to lubricate tap but WD-40 would probably be fine and i saw that suggested in my research.
- I went really slowly and backed out every 1/4 turn or so and cleaned tap and re-oiled. Overkill I'm sure but not hard to do.
My only concern was that I hadn't realized that some taps are 'tapered' and some are 'bottom'. It seems my tap was slightly tapered but reached the correct width (6mm) within about 1/4" of tip. Had the taper been any longer (not sure if this is standard), it would have been a problem because it would have maxed-out against the inside of the seat stay prior to reaching full width.
Thanks again!