Hi Paul,
Since my last pontificating went over so well, I'll do it again. A radiator
boils over when the coolant increases in volume to the point that the
pressure overcomes the pressure limit of the radiator cap...something about
PV=nRT. If a car has an overflow bottle, like a late model B, the air in
the bottle is displaced and absorbs the extra volume so the coolant never
boils over. That's why there's an upper limit on filling the bottle. Early
Bs have an overflow tube so that extra volume is dumped on the ground before
it has a chance to bypass the radiator cap. The problem with this is that
the coolant is then gone for good and doesn't get sucked back in like it
would if it went into a bottle. So to answer your question, Bs with the
correct parts should never boil over.
But of course that's not the whole story, at least not from me. Boiling
over doesn't automatically cause engine damage. If your car does boil over
or run too hot, the best thing you can do is pull over to the side of the
road, leave it running, and pour water slowly over the top of the radiator.
This will cool down the engine enough so that you can either turn off the
engine or add water to the radiator with the car running. If you turn off
an overheated engine, the heat in the coolant that's no longer circulating
will rise to the head and possibly crack it. Of course, if your car is
overheating because coolant can't circulate, like if the thermostat is stuck
shut or the water pump belt is gone, then no amount of water over the
radiator is going to do you any good and you should turn off the engine as
soon as possible.
The difference between running temperatures for early and late Bs is partly
the tuning and partly the thermostat. When Bs had to pass more stringent
smog laws, they were set to run with the timing retarded which causes a car
to run hotter. Also, the thermostats were changed from 180 degrees to 190
degrees. At the same time, the temperature gauges were changed to have no
numbers on them. MG was worried that people would be upset if they knew
that their cars were always running above the boiling point of water. With
a 180 degree thermostat, my '67 runs at 185 and with a 160 degree
thermostat, my car runs at 185. It just takes longer to get there.
Denise Thorpe
PaulH. asked:
The question still stands, just how hot does it have to be to have an MGB in
good condition boil over?
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