A multi-lock adapter used to inject high-pressure well stimulation fluids through an independent screwed wellhead includes an adapter pin having a central passageway with an internal diameter at least as large as a passageway through the wellhead. A lockdown flange secures the adapter pin to a casing mandrel of the wellhead. The lockdown flange ensures that stress on connection points to the screwed independent wellhead due to elevated fluid pressures used for well stimulation procedures does not exceed engineered specifications.