Hi AVB. I hadn't heard of them but since you mentioned it I did a google search and there are lots on ebay - specifically for 1/2" Tecumseh threads too. As usual I learn these things hen it's too late!
...and Norm, I think the thread on the crank is OK but I can't find a nut to fit it. Bunnings only had coarse threaded 1/2 inch nuts but I think this one is 20 tpi.
