Yeah like an alt bracket to engine was SAE, but the alt to bracket was metric etc etc… That sucked on the 80’s cars… Makes it real fun when pulling and building the engine, putting it back together was a pain if you didn’t keep the bolt/nuts with the part they came out/off of…
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I am surprised a 1999 Toyota would be this way. Anyway, I think maybe SAE wrenches should come with something like 12.7 mm for a half inch marked. It almost seems like if everything was on the same page, there would be less room for confusion, even if decimals seemed wonky. That way both systems could be used interchangeably. At least 12 and 13mm are way different on paper than 1/2 so maybe that would cause people to think more and realize they are on different systems. Honestly, the way people think (or more DON’T THINK) might make this even more problematic. I know I won’t solve that problem either.
I just thought of another situation like this that is very car related in nature. Look at tire sidewall markings. Not only do they mix metric and standard, they include a ratio as well. Tire Size Explained: What the Numbers Mean | Les Schwab.
As I’m approaching 70 I have a hard enough time reading those small numbers on a socket.
I still think it’s easier to keep them separate. When I’m working on a GM vehicle, I know it’s all SAE. Asian - Metric. I haven’t seen the mixing of sizes in years.
I worked as a mechanic back in the early 70’s to help pay for college. All American vehicles. Then I started to work on some foreign vehicles and now had to buy new sockets and wrenches. At least they made Metric sockets for my 1/2", 3/8" and 1/4" wrenches. After 40 years of marriage my wife finally understands that when I buy a new tool to do a job - it saves us money. I may even only use that tool ONCE - Still saved us money.
Maybe I said it already but I paint a blue line or dot on all my metric sockets and red on the sae. Just helps to sort them back again without having to read the markings. The cheaper the sockets seems like the shallower the markings. Haven’t found it necessary to paint the wrenches. I use brown for odd ball stuff like square for garage door springs. If I ever bought wentworth, I’d use green. Then I’m out of colors.
I think a lot of the mix bag of sizes between SAE, Metric, Whitworth, etc…, is the result of this “Global Economy.” Manufacturers build so few of their smaller parts and they contract them out and then; maybe, perform the final assembly… Loosey-goosey written contracts may not specify the type of screw or bolt and just leave it up to contractor, or they just buy whatever is on the market… So a starter is all metric, but it is later attached to the engine by SAE bolts…
What is REALLY irritating and stupid, is, for an example, a 1985 Mercedes Benz 500SEL used the exact same A/C Compressor as a 1970 Chevy Chevelle with A/C, the old A6 compressor…
Back in the mid to late 90’s, the Chevy A6 compressor with SAE bolt holes tapped into it cost around $100.00, but the MB A6 compressor with the metric bolt holes tapped into it for mounting and manifold hose cost around $400.00, WTH??? Funny a metric tap and die set cost about the same as a SAE tap and die set… I have both… I guess I can charge 4X the amount to tap a metric hole than a SAE hole… 
Not unique. I had a 94 Ford Ranger. Most of the body was SAE but the engine was metric. That was during Ford’s partneship with Mazda, something I can still see in my 07 Miata whose transmission is made by Ford. I think the fasteners are all metric but the transmission still shifts most smoothly using Ford fluid, one of the secrets shared on Miata fan sites.
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Can I have my wife talk to yours?
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GM has been metric since the X cars in the 80s. Not ALL metric… 
… Any new tooling was metric. The stuff that did not change was still English which is why the smallblock Chevy engine stayed English… but with metric accessories 
making you keep both sets of wrenches handy.
My 93 Saturn was 100% metric. My 2004 Chevy truck has English bolts holding the rearend cover on. Oddly enough, my 2013 Mustang has the same English rearend cover bolts.
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I guess another one is how we all have sockets with a standard measurement for the drive like 1/4, 3/8, 1/2 inch, etc. but the socket itself can be metric or SAE.
I was completely unaware of the Whitworth standard but it looks like it would mainly be used in older industrial stuff. I have no clue if this is still common or not. Is it just the threads for tooling or are the heads different as well?
Old British cars and motorcycles also used Whitworth. The heads are different too.
I have also been curious about that.
It’s like tires for cars. The world uses wheel diameters sized in inches, tire widths sized in mm.
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Interesting. I’m curious about the ratchets. Is 1/4, 3/8 and 1/2 inch standard through out the world?
From what I can see and read about, 1/4, 3/8, 1/2, 3/4 and 1 inch (maybe even bigger) square socket drivers for both metric and SAE are standard world wide…
Michelin did attempt to get away from wheel sizes in inches, they were used on a number of cars (including the Mustang), but faded away:
Michelin’s Infamous TRX Tire Was Way More Clever Than You Realize (roadandtrack.com)
There are just some things that engineers need to leave alone and stop trying to reinvent the wheel (pun intended)…
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They are all exactly the specs specified based on whatever tooling, new or old, worn or not, quality or guesstimated at hand at time of manufacturer…
And other engineering fails…
