[lateapex911]

I don't know if there is a standard...
But, the way I size things is to get my thread pitch tool, which came with my tap and die set. It's a little assortment of what look to be mini saw blades, and you try each one until you get a good fit, it's obvious when it matches.

So, those are available at hardware stores, Harbor freight (maybe not just the thread tool, but part of a tap/die kit, which could be cheap) or in a day or two from the grey van people.

The next thing is to measure the shaft. I use a micrometer. Those are also available at Harbor freight, although you can get good ones online, and compare the diameter to online bolt size charts.
My micrometer is digital, and saves me time by switching between mm, decimal inch and fractional inch...handy

I have a Tru Value store thats family owned locally that has a great hardware assortment. WAY better than the home improvement stores, who, even if they have a bin for a 22mm x 1.0 nut they sure won't have the nut in that bin, if in any bin.

The reason yours got rethreaded is because the 22nut (actually M16 x 1.0) is very unusual*, but is sold by our host and others.

Finally ...that set up is unique...it often includes a spring washer, plus the super thin spacer washers. I assume you just lost the nut?

Wild guessing, 5/8" is slightly smaller (IIRC) than M16, so there's a possibility somebody was naughty...so don't exclude SAE sizing...you never know, our cars have been around the block!

I'm having similar issues...lost my nut (After plating) M16 x 1.0 won't fit, nothing fits. Measures M16 x 1.0 though. Driving me nuts.

*Typically, IIRC, a M16 x XX nut is a 24mm wrench size. The nut on the alternator is actually a thinner than "normal" nut, called a jamb nut. Not sure why the stock version is a 22 wrench size.