Intuit said:
Standard heads are unusual on vehicles. As I've stated, it's the only standard head on the vehicle that I'm aware of. 15mm doesn't fit, 16mm would round it off. What else would it be other than standard. Drain pan bolt heads may have well been one of the last hold-outs on a long dead era. Frankly, can't say I miss those evil fractions. With exception to that 5/8 box wrench, all of my standard tools are shiny brand-new... looking.
Just for the record, 5/8" and 16mm wrenches can be used interchangeably. I do it all the time.
I haven't done the math, but 15.88mm sounds about right for 5/8". That .12mm is not enough to cause a 'rounded off' head (assuming the wrench and bolt head were both in good condition) and is well within manufacturing tolerances.
You'll find that bolt-head sizes tend to be a tiny bit smaller than 'advertised' to make up for manufacturing tolerances for the wide variety of tools. There also has to be a bit of 'wiggle-room' so that these tools can easily be installed and removed. If the bolt head happened to be 16.01mm and the wrench happened to be 15.99mm, then it would be difficult to get the wrench in place, and also difficult to get it removed from the bolt due to the interference fit.
Likely, a combination of a worn (or cheap Chinese-made) wrench, worn bolt head, and/or possibly not having the wrench properly squared on the bolt head is what caused this to round off.
Technically, this *IS* a 16mm bolt head (even
Autozone lists it as a 14mmX1.5mm thread and a 16mm head), but even so, they will NEVER put a standard sized head on a metric-threaded bolt.
Regardless of the debate, I do tend to prefer to use a 5/8" wrench on the 16mm drain plugs as they do fit a bit more snugly, and can compensate for a worn bolt-head.