intake/exhaust mounting, what am I missing?

Dne007 said

so, just a regular nut? seems like it would back off, but then seems like they would all back off without a lock washer.

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Correct. Just a standard fine thread nut. It won't back off. That conical washer acts as a "spring" and compresses slightly when the nut is torqued. That, plus the slight stretch of the stud keeps the nut tight. None of them need lock washers.