300tdi top hose

what’s the internal diameter of the top hose please thanks in advance

32mm on the smaller radiator side on the D1, assume the same in other applications. If you're thinking about adding a coolant gauge, you can also tap a 1/8npt thread into the radiator plug and plumb a sensor there (needs to be brass of course). This turns out much neater, and you don't need an earth strap hanging off if the rad has metal end tanks to boot. (like you do with a pipe t-connector style one)