Material must be too brittle or they will break the center when the slightest lateral pressure is exerted on it, for example when the surface gripped by the head is not exactly perpendicular to the axis of the screw. If the screw flexes as you tighten it, it will lock. I've never had a problem with broken GRK case screws, but they weren't a 2" size so I gave them a try.
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