I recenlty took apart a subframe from a W124 donor car. I found that the only way to get the XZN bolts off was to first soak the head in PB Blaster or spray on wheel cleaner and then to use a small screwdriver with a rag to get all the crud out of the bolt head recess. Otherwise, the XZN tool would not go in far enough to sit properly and the slipping would happen. On a couple, I even tapped the tool in a bit with a small hammer or a ratchet. But they all came out intact with patience.

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I never go far without a little Big Star

1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k