how big is your lathe if it only takes 10mm tooling? to bore 188mm you're going to need at least 400mm (16") movement on the toolholder unless you use a series of tools or move the tool in the toolholder either of which has implications on the accuracy of the bore dia end to end (and how would you measure it?). To use one fixed tool implies something capable of 18 - 20" between centres, your average 7x14 isn't going to cut it (excuse the pun)!