Aero Precision Fix it Kit, Includes Takedown Pin Detent (4), Takedown Pin Spring (4), Bolt Catch Roll Pin (2), Trigger/Hammer Pin (2), Safety Selector Spring (2), Safety Selector Detent (2), Buffer Retainer (2), Buffer Retainer Spring (2), Disconnector Spring (2), and 4-40 Set Screw (for Threaded Takedown Pin Detent Hole) (2) APRH101627 Did you launch a detent and spring across the garage during your build and now you can't find it? Aero's Fix-It Kit gets you the most commonly lost parts to finish your build. All parts in the Fix-It kit are compatible with AR15" M4E1 and M5 builds.