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The purpose of these illustrations is to demonstrate the long term affects of using unleaded gas without hardened valve seats machined and installed in cast iron heads.
Our engines were specified and built to burn leaded gas before the 1980's. Without lead, exhaust gasses set up hot spots at the valve seat mating surfaces. When the valves closed they actually welded themselves to the iron head. When the valve opened again, this tiny weld broke free, which caused erosion. After years of this reaction, the valves eventually pull up into the head.
This set of heads was waiting to be discarded. They were original to a 1973 F-100 with a 390. This engine was very low compression, but a work horse. The odometer showed 78,000 miles. Notice the difference between the middle and the end exhaust valves (the smaller ones). Typically, the end cylinders run leaner and hotter.
I took pictures of all the combustion chambers just to show the whole story.
This one is pretty bad. Eventually, the rocker arm must hold the valve open because there is no more lash adjustment.
Today's engines use stainless steel valves and very hard seats formed from powdered metal.