Machinery's Handbook shows drills up to 27.0 mm and 1 1/16" take 3MT,
and those from 27.50 mm and 1 5/64" upwards take 4MT.
Since I've got the book out and found the right page, for anyone else
who might be interested, here are the maximum sizes for each MT:
1MT --- 11.8 mm/15/32"
2MT --- 19.5 mm/25/32"
3MT --- 27.0 mm/1 1/16"
4MT --- 38.0 mm/1 1/2"
5MT --- 76.0 mm/3"
This is for regular shank drills, some longer or shorter ones switch at
a larger or smaller diameter.
Of course, this is a largely US-oriented book, so Europe may do it
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