Hi Beaufighter VI

The Whitley parts catalogue states AM stores ref: 27A/1465 – thermostat (Double Merlin), type B No. BGA 3891D – this may or may not be helpful!

Looking at the cooling system schematic – the diamater of the pipe from the thermostat to the radiator is 2″ (I am assuming both input and output are the same as it doesn’t give the diamater from header tank to thermostat).

The thermostat bypass outlet is a smaller diameter and connects to a 1.5″ bore pipe.

I hope that helps

Elliott