intake manifold plugs

Just to avoid future confusion, a tapered pipe thread is not usually designated as 3/8-18. Just say it's 3/8 NPT, or even 3/8 pipe thread, and everybody will know what you're taking about. For reference, the biggest part of a 3/8 NPT is around .638", 1/4 NPT is around .502", 1/8 NPT is around .380". Here is a chart: National pipe thread - Wikipedia