Looks good to me. I can knock a new one up with those details altered and put it into the How To that you write.

Do you cut the threads with single point on the lathe or use taps/dies?

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the world is a book, and those who do not travel read only a page
66 Mini Minor sponsored by http://www.lifeonthehedge.com.au/ The Dog Harness Specialists
It was a pleasure ausmini. I'll miss all you misfits and reprobates

Thanks Michael.
I'll write something up tonight.

My lathe is a 1950s Myford Super 7 bench lathe. A whole 3-1/2" centre height. And it's not in new condition.

I only screwcut threads here if I have to. I use a 5/8 Whitworth tap in the tailstock drill chuck, and a 3/4 UNF dienut to cut the threads. I square the dienut against the Jacobs chuck to keep it square.

Edit-
Michael,
Blog now created in the how to forum.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Simon I have checked the outside diameter of a 5/8-11 UNC (similar to 5/8W) and it's around 3/4" free diameter. So already too big to take this adaptor.
I suggest smear a little silicone gasket maker on the sump plug threads before fitting each time.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.