That is correct-strange isn't it!!!
Strange?
I believe he is referring to the combination of Jap/metric motor with standard size crank bolt.

Got what the owners manual stated 5/16-24 from home depot, but it didn't work. Took the motor with me and 5/16 fine thread fit.
...But, it's a chinese engine...
Now that's strange...
Got what the owners manual stated 5/16-24 from home depot, but it didn't work. Took the motor with me and 5/16 fine thread fit.
5/16"-24 means the size of the bolt is 5/16" and the -24 means there are 24 threads per inch on the bolt so making it a fine thread bolt.
On a normal 5/16" bolt there is 18 threads per inch making it a coarse thread.

As far as the SAE/Metric thing goes- it's because the engine was optioned for the north american market. If you got a european engine, you'd have gotten a metric PTO shaft to go with your metric bolt size.
Could I have gotten the wrong bolt or is it hard to thread a new bolt in a new engine at first?
I counted the threads, it was an 18, not 24. Next time I'll look myself for the bolts.