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What do these air ports actually do?
Is there any exhaust pressure in them that they would leak?

Trying to figure out how critical the seal is between the air port threaded plug and the head itself.
 

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I'm speaking from a stock 5.0 perspective, but assume the X-303 heads are the same.

My stock '86 setup has a hard tube that connects both "air ports" at the back of the heads and are then piped to the air pump driven off the fan/crank serpentine belt. It's there for emissions. The air pump forces "fresh air" into the back of the heads. There is a small hole/tube that runs the entire length of the head. (In fact, the heads can go on either way - the front "air ports" being blocked-off by the accessory studs and such.)

Anyway - air is "injected" into the back of the heads and along those "tubes", where there is a small hole and "hump" tapped into each exhaust port. This "fresh air" allows for more/any unburnt fuel to ignite in the exhaust tubes/system making less emissions.

At least - that's the way I understand it. The only side affect of not having the holes sealed would be increase exhaust noise and some exhaust gasses (from the back of the heads.)
 
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