Exhaust manifold lock nut. Left.GAS. Right.GAS. A solid metal or plastic bLock with a threaded hole in the center which is used to tighten or adjust the Exhaust Manifold Heat Shield. Exhaust Manifold Nut. Part cannot be reused/reinstalled. Power Brake Booster Nut. Cover. Heat. HexAGON. Power Brake Booster.