<VV> dropped valve seats

[HallGrenn at aol.com]

Tim,
 
Based on a conversation I had with an experienced machinist who did a lot  
of air cooled heads (motorcycles, Porsches, aircraft engines) I would 
emphasize  Kevin's comment about valve float and how it reduces heat transfer 
because the  valve seats don't stay in contact witht the seat as long.  It is 
exactly  what the machinist said about air cooled heads with inserts.  At the 
risk  of tempting fate, I have only had one dropped seat (in a 140) so I 
don't have  much personal experience, but the machinist's rationale made a lot  
of sense then and now.
 
Bob Hall
Group Corvair
Corvanatics
CORSA 
 
In a message dated 5/10/2010 11:45:44 AM Eastern Daylight Time,  
wrokit at hotmail.com writes:

Timothy-  your experience is why you should have the seats and guides 
replaced on any  head that you do not know the history of. If you do replace the 
seats and  guides, have the "deep"

seats installed, they provide alot  more grip (because of the extra depth) 
than the stock seats do. Also, most "in  the know" corvair head rebuilders 
put these seats in with a higher  than

stock interference, which gives more grip, but also  gives somewhat better 
heat transfer from the seat to the head. Also, it is a  very good idea to 
set the valve springs up

with 10lbs more  preload, as a underappreciated source of dropped valve 
seats is valve float.  The extra preload also improves heat transfer as the 
valve is in contact with  its seat

longer.

Kevin  Nash