The MG is essentially all SAE with the odd BSW/BSF fastener thrown in. When I find a metric thread, it's most likely because someone who worked on the car in the past put it there. There are a few other oddballs in there, such as British Standard Brass on the carburetor jets. Most of the hydraulics are British pipe, and the heads are generally either a Whitworth or SAE size. Interestingly enough, many folks will use a 7/16" SAE wrench on the distributor clamp, and often end up with the clamp either not tight enough or too tight. I've sold people on the value of having a set of Whitworth wrenches when I hand them a 3/16" Whitworth wrench and they see that it fits this bolt perfectly. The nut being tightened is technically 1/4" BSF, but the head is the same size as 3/16" BSW.
Interestingly enough, the XPAG and XPEG series MG engines were built machinery imported from France. Virtually all of the engine screws are metric, but have a BSW head. The official reason given at the time was that the mechanics would have to buy all new tools if they kept the metric heads, but one of the finest British mechanics in the US is fond of saying that the real reason was "We're British and we'll be gosh durned if the French are going to tell us what to do."
The fasteners on my Lincoln are overwhelmingly metric, although there are some SAE ones thrown in there. Interestingly enough, as an example, the bolts holding the COPs in place are SAE but they are accessed by removing covers that are each held down with 6 metric head bolts
. I currently tend to three other American cars-two other Lincolns and a Buick-and the fasteners on them are overwhelmingly metric also. I'm not sure if I've found an SAE fastener on the Buick. Interestingly enough, I changed the oil in my dad's MKS a few weeks ago, and the the splash guard bolts were all SAE. Of course, Torx is also common these days.
I've also been looking after a '97 Geo Prizm more recently, which needless to say is all metric considering that it's a Toyota. I think the only GM part on that car might be the steering wheel logo-it's lived a hard life and doesn't even have the grille badge anymore
. I'm trying to keep it going at least until my friend who owns it finishes her PhD in May-she knows the writing is on the wall.