Mill Head Surface
With today's heads being border line high performance, a lot of stress is placed on the head gaskets. Today's engines use detonation sensors to back the timing up when it senses detonation. By running them on the verge of detonation they burn cleaner and perform better, but detonation is very hard on engine components, especially head gaskets.
Today's engines are usually cast iron blocks with aluminum heads. These two metals have very different rates of expansion. This causes the two surfaces to slide against each other. If the two surfaces are not properly machined the head gasket will fail prematurely.
We measure the Ra finish on our heads and blocks to make sure they are within OEM specs.
It is very important to straighten OHC engines before milling them.