Hi,
Thanks for your help, it's reassuring to know a bit of leeway shouldn't make too much difference.
The drawings are useful - I've saved a copy as they may come in handy for other projects at a later date.
I was gutted that I lost the original valve, I bought the engine from someone else and although it was in a poor cosmetic state it's an excellent runner and appears to be well machined so I was really hoping to make a valve that will allow it to run equally well.
I lost the part when stripping it all down for painting and even after moving my lathe, mill, searching the contents of the Hoover etc it appears to have vanished never to be seen again.
Peter.