These double adjustable upper control arms allow adjustment of the pinion angle on 1979-04 Mustangs without having to remove the control arm to rotate the heim for each incremental adjustment. Simply loosen the locknuts, turn the adjuster to establish the pinion angle then re-tighten the locknuts with basic hand tools. Saves time and your knuckles. Will not fit 1999-04 Cobra with IRS.