adrie.degraaff wrote:I would weld it back to size and make a new thread.
Personally, I would avoid doing that because the heat will distort the end of the crankshaft and cause it not to run true. While rebuilding material with welding is OK on some components and in some circumstances, it's best not to have the engine sprocket wobbling.
It's not so much that metal has been lost from those threads, it's that the crests have been pulled down into the roots. Before considering drastic solutions, like reducing the diameter to 5/8", I would make a "thread rolling" tool similar in design to a lathe's knurling attachment but with a narrow 55° wheel instead of wide knurlers. Set the lathe at 20 tpi, locate the wheel in one of the good grooves, and manually work it back and forth to push the metal back where it belongs.
Thread rolling with multiples wheels is a common way of making threads in the first place. For your crankshaft a simple, easily made tool, would do the job.