MGB GT overheating only on motorway.

with the rad.cap is off,look inside.how do the fins look ****??...have you serviced the radiator?did it over heat quicker, slower or the same going down MOTORWAY with the thermostat out??? do you add water all the time? when hot are the rad.hoses staying open and not sucking closed..did you get all of the air out of the system? PUT a new 160o thermostat in it.. required to keep water in the radiator longer !! add 1 bottle of water wetter..to the rad water...also dont run 100% antifreeze/coolant 40% af 60%water..

A simple thing to check before getting into more expensive solutions is the lower radiator hose. It may be collapsing under higher engine RPMs and restricting coolant flow. There should be a spring in the hose to prevent that, no idea what MGB used back in 69 though. After that you'll need to look for restrictions in coolant flow like worn water pump, gunked up radiator, etc.

Probably need a cooling system service. I would probably install a HD radiator and maybe an electric fan (will help around town). Use the product Water Wetter in the coolant which will help reduce operating temps. Tune it up because an engine out of tune can run hot.

I installed a new radiator and water pump. That fixed the problem for me. Check your belts too!