In my case - the bolt heads were smaller than normal for the thread, and the bolt heads were flanged, plus shallow. I began trying to budge them with sockets, then tried heat, but wary of damaging the strut. Then finally tried drilling. My drill bits, would not even mark it. I finally resorted to the angle grinder, and the clamp opened up with a bang. The head section almost fell out, and the threaded section came out with a little heat, then quenching with penetrating fluid, and the use of Mole grips.
