@Steve: The threading bar is a SIR S16x07 M08 from
AKKO. Needs a minimum 11 mm id hole. They also sell on eBay (Germany). The insert is a 08 IR 0,5 ISO BMA from
www.bachgmbh.de.
Another german
supplier (they don't seem to have any englich page, am sorry) sell threading bars for holes down to 6.2 mm (1/4").
Stefan