Am I being dumb ?

[Neale]

Ok Neale thanks for the reply, I'm trying to get to a point where I can screw in a 3/8" bolt to the bush after I've ground the head off and fit it into a chuck capable of taking up to 13mm. if that makes sense ?

Sorry but I have to ask again - what 3/8" thread? If it's an old bolt, then it might be (imperial) BSF or BSW, newer might be (unified) UNF or UNC. Might, just, if it's a bit sloppy on its OD, be M10, depending on how accurately you have measured it. You might end up having to buy a matching tap and making it yourself, unfortunately, unless you have a kind forum member nearby with a lathe and good set of taps!

Edit - I see that you've more or less reached that conclusion!