First the tips of the blades warp.  Then, after I trim a bit off so
that they won't hit the cowling, the warp a bit more over the course
of a week or two.  Then, after a longer time, the nut breaks off the
fan, or vice versa, I guess.  The fan blades actually have bubble-like
depressions in them.

I am considering the possibility that this may be exhaust heat, in one
form or another, either radiant or an exhaust gas leak.  I did replace
an exhaust manifold gasket between the first and second times this
happened.  The exhaust leak would handily explain the first melting
fan, but not the second time, as the exhaust is now quiet and
therefore not presumably leaking..

Still, exhaust heat is a source of engine heat that is hot enough to
do this, and it would explain why the coolant temperature can be
normal while the fan is melting.  So, it is a very real avenue to
explore.

Hamish
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