What kind of plugs for exhaust manifolds?

[raceman6135]

The A.I.R. tubes are a straight thread because the tube's sealing is accomplished via the 45-degree mating surfaces of the tube to the "cone" inside of the exhaust manifold.

If you can find a straight thread plug, go for it, but when I had trouble finding one, I just used a brass NPT (national pipe taper) plug. The brass is soft enough that as you tighten it, it will deform to the threads in the exhaust manifold and will seal without the need of any threadlocker, or paste, etc.

Since I knew I'd never be removing the plugs once installed, it didn't bother me that I forced the NPT plug into the straight thread manifold.