> Is this opinion or fact? If factual where does one find that
> reference? I'd
> like to know the origin of this statement.
Seems self-evident to me. Without the bypass, the water doesn't circulate
at all when the thermostat is fully closed. With no circulation, you're
going to get hot spots. The hottest spots will be in the head near the
exhaust valves. Since water is actually a pretty lousy conductor of heat,
the water next to the valves can easily reach boiling before the water next
to the thermostat gets hot enough for the thermostat to open. When that
water boils, it will be lifted away from the head surface, removing the heat
sink and allowing the metal to get _very_ hot (remember there are gases on
the other side of that thin wall of cast iron that may be over 1000F). When
the water does start to circulate, the area will be bathed in relatively
cool water from inside the block (and the radiator, which is still room
temperature at this point).
Hot cast iron + cold water = cracks. Maybe not every time, but it's a
definite chance.
Randall
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