<VV> Valve adjustment & dropping seats

[Craig Nicol]

I'm having a hard time getting my head (pun intended) wrapped around the
notion that improper valve adjustment (i.e. the valve not being seated
enough) having much to do with dropping a valve seat.  For one thing, the
classic issue is an overheated valve, not an overheated seat.  In the
overheated valve scenario, if the valve is not closing properly, the valve
burns at the edge or seizes in the guide; seats aren't the issue.  

Another thing, intake seats are the ones that sometimes fall out on the 140
and the valve and seat on that side runs relatively cool, what with the
evaporation of fuel and cool gases flowing through the area. That's why
intake valves are made of less exotic materials.

Here's what I've come to understand and believe about dropping seats:
After a high load event, such as climbing a hill, the cylinder head becomes
very hot and expands accordingly.  The seat, being steel, expands less. This
creates a low seat-retention condition. Now, change to the coast side of the
hill or even simple deceleration. Incoming air quickly cools the seat and
intake vacuum is at the highest level.  With the further cooling of the
intake seat and little change in head temperature, seat retention is even
less. This very low seat retention condition combined with high vacuum sucks
the seat right out of the head.  Sudden misfire on a cylinder when an engine
is at high load can create similar conditions where the seat is rapidly
chilled and sucked out. 

Because the 140 valve seat is larger than a 110 valve seat, AND the 140
cylinder head runs hotter than a 110, a 140 is more likely to drop a seat.
Of course there have been occasional exhaust seat failures on all engines,
but 140 intake failures predominate. 

Tight valves create a low power engine that burns valves. Loose valves
hammer and damage the valve train and create more intake vacuum (less
overlap) but I don't see a relationship between loose valves and dropping
seats either.

Open to thoughts one way or the other.
Craig Nicol