[Richard Coutts-Smith]

Mike and Phil, thanks for your thoughts on this subject.
It will be a couple of weeks before I am back to this Champ (Mike you need to check your spelling, Chump indeed!)
Not sure that a diameter would be much help as it may well have been machined to fit, or at least to help the bog hold it in place.
I have a pic from another angle, but probably does not add much.
Rich.