Bob, Thanks for your reply. Interesting to see the Daimler 15 thermostat housing. The Lanchester 15/18 just pre-dates the standard fitting of thermostats, so any that have / had it are modifications. I'm not sure whether to persist. One thought is that maybe the radiator water level was too high (although I don't think so). I'm also wonder if once the water started whizzing out of the overflow, it then worked like a syphon. The other thought is an airlock, although I was careful to fill the cooling system, first through the impellor housing outlet, then after fitting the top hoses, through the radiator filler. One thing is for sure........I would want this happening whilst on the road!. I hope your engine rebuild is moving on well. All the best, Ranald