The 2011+ Ford 5.0 Coyote motors are fitted with a plastic intake manifold from the factory which is known to deform the injector bungs over time.  While the bungs are circular when new there have been numerous instances of the holes becoming more oblong and thus not allowing some o-rings to seal properly. Injector Dynamics offers a special o-ring known to work in this instance, and all 5.0 2011+ sets of injectors are sold with this o-ring installed on the lower adapter.  As of August 2019, Injector Dynamics will also be sending an additional set of o-rings on the side that are used in a typical 14mm bore with our lower adapter.

Below you see our lower adapter complete and then the two available o-rings along side. The o-ring on the left is a thicker design which allows sealing in most cases where a slight deformation has occurred.  The o-ring on the right is the o-ring now being sent on the side which will be used in applications where the factory manifold is no longer being used.  Most aftermarket manifolds are not prone to the same deformation issues and this o-ring will most likely be the correct choice. A small amount of lubricant should be used all  o-rings prior to pushing into the bore. 

***As is the case with any fuel injector installation, only attempt to work on the system with a cool engine and check for leaks with the fuel system primed and engine off before driving.***