Years and years ago we were running a dry sump pump and Bill Crosby in New York made me a bolt like you are looking for. Steve (CTR)

Thanks to all for comments... What I really need is a longer bolt... SOmething like 4" long... or a die that I can use to make some threads... Any ideas??

That is correct, it is not a "true Whitworth".  There is no official 5/8-16 size Whitworth thread profile.

The crank taps are called out as "Whitworth form" in a footnote of the factory Workshop Manual in section A.31. 

Various sites claim this the thread form is British Small Arms, not true Whitworth.
Found an MG site that recommended a tap from Toolmex.
https://www.toolmex.com/products/search/5-750-6082#7737

As Steve said, it's an odd, non-standard size.  It is 5/8-16 "Whitworth form".  There is no true Whitworth in this size.  This bolt just happens to use the Whitworth thread profile.

There are taps for re-threading this to size if you need them.  However, you are not going to go down to the hardware store and buy a correct tap or bolt for the crank.

Yes it is a crazy size and to make it better a strange thread profile as well. There have been threads about the tap and they can be bought but are a little expensive. The flywheel bolt has the same thread as well. Steve (CTR)

Anybody know the diameter of the bolt and the number of threads per inch is used on the standard mini front damper bolt?? It is an unusual size.. It isn't coarse thread and it isn't fine thread.... Know what it is?? Is it something like 9/16" - 16???