I was looking at my head (from the combustion chamber side) and noticed a hole beside each exhaust valve guide, they are connected to a galley that runs the length of the head, with brass plugs at each end. What is or was its purpose?

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Air from a belt-driven air pump or an aspirator pipe and valve was piped to the master connection (at the rear of the manifold mounting surface on the head) and distributed to these holes you found. This air caused unburned fuel in the exhaust to ignite, reducing tailpipe emissions of unburnt hydrocarbons and Carbon Monoxide. Heads with air injection provisions were introduced in 1972 (for some models only), and are heavier than heads without the air injection provisions.

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