MGB Radiators.

OK, a question for the knowledgable out there.

My B is a 65 Mk1. It has always overheated. show it a picture of a procession or a traffic jam and the temperature gauge ends up in the oil pressure section. I have changed everything several times. Engines rebuilt, water pumps, radiators (this core, that core more fins etc) thermostats, no thermostat, blanking sleave without thermostat, blanking sleave with thermostat... you name it I've done it several times.

MkII B's and later don't have a problem. Everything under the bonnet is the exactly the same except the top radiator hose enters on the right not left.

The other day I had a mate that has a MkII BGT changed his radiator, not because it was failing but the fins were looking a bit ratty. Sadly it's cheaper to get a new radiator than it is to recor the old, but that gave me an opportunity. I have borrowed the old MkII radiator, got it cleaned, tested and painted and I've installed it in my B. A new thermostat housing was needed but it bolted straight in.

Guess what. The engine is staying cool. Hmmmm.  No procession yet but the early signs are quite encouraging.

So my question, WHATS THE DIFFERENCE?  

Jonesy