Had a look at the X5 M rotors & brakes yesterday at the local BMW dealer, the calipers on the M50d actually hold a larger brake pad. The caliper on the M50d is only a twin-piston so it looks like the pad itself is bigger as one would assume the performance of the M50d caliper would not be of the same caliber as the X5 M system, they are a different design.

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X5 M50d White | 20" Rims | Adaptive Drive, Self-Lvl Susp | Act/Steer | HUD | Xenon, Adapt Lights | H-Beam Ast | Rear/Side Camera | Panorama S/roof | Auto Tailgate | Comfort, Active, Ventilated, Heating, Lumbar, Navada Perforated Leather Seats | Comfort Access | M-Steer Wheel w Pdle Shift |Sports 8-Speed | Active Crus Stop/Go | Proff Nav + TV, DAB | 16 Spk Ind Audio | USB |Internet-eMail-SMS-Apps | 4 Zone Air | Speed D/play | Towbar | 20% Tint | 5mm Castor Bushes | Yellow Calipers, Ceramic Pads |