24 Armstrong HeliCoil Screwlock 3591-4CNx.250 Inserts 1/4" x28x1D UNF HC311. 304 Stainless Inserts. The helicoil® screwlock bears the same benefits regarding threads as the helicoil® free running insert. additionally an integrated screw clamping area serves as a screwlock. locking of the screw is achieved by one or several polygonal-shaped threads that clamp the flanks of the bolt. The result is a flexible and resilient frictional locking. The achieved clamping torques are comparable with Din 267 Part 15. iSo 2320 or can also be adapted to your individual application.