<VV> Valve seats - Corvair heads

[Sethracer at aol.com]

In a message dated 6/14/2007 2:49:20 P.M. Pacific Daylight Time,  
jwilson at unctv.org writes:

What is  it?  How common is it?  Is there a way to prevent it or at 
least  lessen it's likelihood?  Should I live in fear that my engine is  
going to "drop a valve" at any minute?





Jeff - While Aluminum passes heat well enough to wick the heat out  of a 
combustion chamber into the cooling stream - water or air, the  non-ferrous metal 
would break down and deform under the pressure and heat of the  valve seating 
on it. Therefore all manufacturers install inserts for the intake  and exhaust 
valves to  provide adequate life. This is done on ALL  aluminum heads, 
whether water or air cooled. When heated, the aluminum expands  at a greater rate 
than the Steel - Or any other material used in making  the seats. Manufacturers 
use this fact to "shrink" the seats in,  originally. They heat up the head in 
an oven and cool off the seat in the  freezer. They remove the head and drop 
(or press) in the seat. As the head  cools off and the valve seat heats up, 
both reaching room temp, the  expansion and contraction squeezes the valve seat 
in tight. When the head  gets hot in use, the clamping force on the valve seat 
decreases. Since the  Corvair head is air-cooled, the actual temp change of 
the Aluminum material is  much greater than that of V8 heads. The liquid coolant 
flow tends to  stabililize the temp in the head by regularly passing cooled 
liquid into and out  of the head. 
 
If the Corvair head gets too hot, the "shrunken-in" clamping force is  
essentially removed. Say you are climbing a steep hill the motor gets warm, hot  
even. As you crest the hill, you let the motor cool down by coasting down the  
other side.  Now the head is hot, and a cool stream of air-fuel  mixture is 
coming in past the valve and hitting the seat? - That is how they  take valve 
seats out in a shop! They heat-up the head in an oven and shoot cold  air at the 
seat. That is also when they can fall out in use. Of  course, tossing a belt 
just accelerates the heating and increases the likelihood  of a failure. Don't 
be afraid, just be aware of what is going on with your  motor. Remember if you 
climb a long hill and get the motor hot, Stop near the  top of the hill and 
let the motor idle a bit with no load. That will cool it off  a bit and lessen 
the change of a seat departure. An a cylinder head  temperature gage is always 
a good investment. - Seth  Emerson  



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