This page: Transfer Drive Case, Transmission Case (6 Speed Man ZF S6-53, 6 Speed Auto ZF 6HP26 Steptronic)(To(V)9A999999) > Transfer Drive & Case > Powertrain > Discovery 3 - 2005 to 2009 > Discovery > Main Menu > Home > Britcar (UK) Ltd
Suggests it's a ZF S6-53.
ZF say they are good for 600Nm max.
ZF Friedrichshafen AG | Cars | 6-Speed Manual Transmission
Spec sheet here which says 470Nm max for the disco version: http://disco3club.co.uk/gallery/albu.../308_03_02.pdf
I'm wondering if they give a lower torque rating for the same gearset used in a heavier 4wd vehicle?

