AEC engines and cooling problems unfortunatly go together, but not exclusivly for that age of engine builders. Long stroke, wet liners of the day, slow reving and evolving piston and ring technology. This gave many diesel engines a problem with liner erosion caused by several design and material deficency's, Injectors and charge detonation tech was really in its infancy also. Liner wall vibration caused by piston slap and ring stutter amongst other causes, actually causing the coolant to loose contact with the outside of the liner causing hot spots and steam pockets giving poor cooling accentuated by poor water pump impellers and water speed through the system. Along with foreign matter and bits of rust moving around in a confined space. This is where good thermostat and water pressure maintenance can help along with a good clean coolant mix to start with. Looking after these vehicles is a labour of love and persiverance, along with being inquisitive enough to do the detective work trying to find a vehicles history as you bring it up to your standard or repair it. You have done a fantastic job on your vehicles, they are a credit to you and your family. AEC seemed to want to shoot themselfs in the foot with regard to cooling and vibration management right up to the Ergomatic cabed variants. You have to love them though warts and all.