That's make more sense
I did not know about 80 degrees. Actually it seems 82 with +/- 2 degrees tolerance
Thank you !
Some are 800, the Unbrako are said to be 820 +/- 20 as you've mentioned and those from China seem to be 900 !
The countersink bits I have for doing them are 800, with the screws I've been using, they seem to seat just fine,