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18 The Twisted Wedge head, as are all aluminum cylinder heads designed today, is capable of being used on late-model, emissions-controlled 5.0L engines. As such, they have passages for upstream Thermactor air injection. Trick Flow includes two threaded inserts to block off these passages, but you'll need four total to block off the front and the rear (late-model 5.0L only needs the front blocked off). So, order two additional inserts from Trick Flow and save yourself the trouble of finding them locally.