VALVE SEAT RECESSION
Exhaust valve seat recession (VSR) is dependent on the load on the engine. Most damage occurs under high-speed, high-power conditions. Engine operating temperature and valve rotation can accelerate this problem. Lead oxide formed by the combustion of leaded gas prevents valve seat recession by forming thin layers of lead oxides on the valve and seat faces. These layers prevent metal-to-metal contact and eliminate wear of the seat.
Heads cast with a higher nickel content seem to handle the VSR better than other engines, but there is no guarantee. I have seen high nickel V8 heads that had VSR on all four exhaust valves and others that only had a VSR problem on one exhaust valve.
Exhaust valve seats tend to 'work harden' with use. Engines broken-in with leaded fuel may survive quite a while before VSR sets in. But even heads cast with higher nickel content need protection after a new valve job is performed, especially if larger valves are installed. The new valve seat is now located on 'virgin' metal and is especially susceptible to VSR until the seats 'work harden' with use of a leaded (race) gas.
xxHardened exhaust seats prevent valve seat recession.
The exhaust valve seat on the left has suffered from Valve Seat Recession,
while the exhaust valve seat on the right is perfectly fine.
This exhaust valve seat shows the severe damage of valve seat recession.
The seat was sunken in by more than .200".
Hardened exhaust seats installed in this head will prevent valve seat recession.
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