Porting and Polishing of heads
The reason for porting and polishing of heads is:
to increase the "flow" of the ports and runners in the head.
When the factory casts a head, it is made with a mold. The mold has an releasing agent (powder or sand), this allows the cast for the head to be removed from the cooled metal. When the head is removed, the releasing agent leaves a rough surface with small spots of "slag" (bumps of residual metal). These bumps and leftover metal chunks interrupt the flow of the intake and exhaust ports.
To "flow" a head, you smooth out these dimples by porting and polishing certain arias of the heads, hence, increasing the movement of the gasses through the head. Once the head is corrected, it is mounted on a flow bench and a reading is taken to measure how much improvement has been made.
( An in-depth explanation will be provided in a link...)
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