We build these to replace the Factory part number 1311317 - they are made from a solid block of 350 Steel for the best balance of anti warp /flex under heat and load.  
 
The Part fits 3.0 TDV6 on Discovery 4 and the TDV6 2.7 in the Discovery 3, The Part uses your existing balancer and Bolts.
 
Tough Touring Reccommends installation via a qualified Mechanic. 
Please ensure that Locktight is applied to all bolts during the installation. The Factory Pulley belt should also be replaced at the time of upgrade if its showing any signs of wear.