That is a guess.. With metric it will be the diameter of the threaded stock. A nut that fits a 12mm wrench would likely be a 8mm thread diameter. Could be 8mm X 1 mm or 8mm X 1.25mm.. Think 8mm x 1.5mm is "course".
Get a full metric chaser tap and die set and a metric thread cutting set. Well worth the investment. Spining a chaser die or tap together with the hardware by hand will give you a positive ID.. It also makes cleaning up hardware or tapped major assemblies easy.
I've since replaced the exhaust to 3" mild stainless with custom aluminized Y pipe and new O2 bungs... Now my O2 sensors are showing as fried so I'm swapping them out with a set I bought way back when from Cruiserdan. Always something.
I would post up pics of the exhaust, but its not 'pretty' under my salted road driven truck. Mild stainless looks like junk pretty quick.