I had a senior moment and worked out metric pitch by dividing 20 TPI by 25.6 rather than vice versa. Silly me!
The bolt I found that fitted came out of my metric bolts pot but could easily have strayed. I will check the head with both AF and metric spanners. Not conclusive, I know, but interesting to know.
7/16" works out as 11.2" The bolt that seemed a good fit was 10.8" The thread in the hubs are a bit grubby but it didn't feel as though it was under size.
I will have a gentle go with a 7/16UNF tap to see if I can clean the thread without removing new material.
Thank you, Geoff. Very kind of you and I will send you an email.
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