i rebuilt my own box mate took all the internals out an took them to an engineers shop in warrington,he does a lot with formular ford,he take your internal to a place in north wales called british aerospace were they build planes and all that etc an they do the shot peening and he simpley polishes the gear set and crown wheel for you ,think it cost 55+vat,then i took the viscous diff to bara motor sport in bromsgrove and he rebuilt the diff and put a higher viscousity fluid in it ,this was tighten to 120nm,very tight diff that cost 300 hope this helps