Great, I have some thin wall tube, in my usfull box.
Just happens to be 19mm od 16mm Id.
Thanks very much for the quick info, I'm actually doing the job right now 🙃
I guessed at 34mm looking at the nut and hole for the split pin.
I might try that, and check the caster.
As I have just fitted lowering springs of minus 2 inch, it will have more caster now.