Yeah Blah, the 80s GM cars were the worst for their standard/metric mix and match fun. I remember an 80 Olds Omega I had with the body & suspension hardware in standard sizes and the engine was all metric, whatta mindfuck that was to work on!

Here's a fun fact I learned when workin with bearings. All bearings and seals are made to metric specs, even ones that match up to inch sizes. Bearing catalogs here will typically show the specs in inch sizes because most people think in these terms but they're actually built to a metric spec.

Here's another interesting difference between inch and metric sizes when it comes to bolts. Inch size bolts like a 1/4 - 20 means the diameter is 1/4" with 20 threads per inch. A close metric comparison would be a 6mm X 1.0 which means the bolt is a 6mm diameter with a distance of 1.0mm from crest to crest of the threads.

GAK & 'berto, most of our material items here are measured in inches like in steel bar and flat stock, pipe, wood, ducting materials and such though you do come across some variations in that from time to time. If however you need a piece of 12mm shafting for a part on a machine you typically have few sources to find it available from, while you can find 1/2" rod stock at any steel supplier. And, if you're dealing with a machine shop they'll want the specs expressed in inches and thousandths of an inch, though some are conversant in metric figures, most prefer inches, which works great for me, so ...

Like John Prine said : "Ooh baby, It's a big old goofy world"