Does anyone know of a bottom ball joint type that will fit into the lower taper on the Sierra front upright without modifying the taper?
Luego suggested one with part number TC 72 ( an AP number) but the socket in the hub needs to be enlarged to get the proper depth of engagement.
Westfield parts department say that they enlarge the hole in the stub axle for their SDV cars, and use an insert. The ball joint they use is the Maxi one, as used on the Cortina uprights. Unfortunately they will only modify the uprights for their kit customers. I am sure this topic has been discused before but I have not found the answer yet.

Also, has anyone noticed that the lower taper on the Sierra upright is offset rearwards by about 6mm, relative to the upper hole, which clamps the strut on the Sierra?
This will influence effective castor angles, since on most uprights a line between the upper and lower ball joint centres would pass through the stub axle cente.
Any comments please?

mk will bore the hubs for you

Some have said that the BJ off a MK2 astra fits in direct, though that is hearsay cos i've never done it. Also that BJ isnt designed to take the load of double wishbone life (on a strut car it takes much less weight than with double bones)

Best bet is to bore out the bottom hole on the hub, easy peasy. er, just get some one to do it for you! MK did mine for free when i bought some bits off them, so maybe try them. Then u can use a cortina ball joint that takes the load well. They sell a nice insert for the top as well, that takes a transit rod end perfectly.

As for castor issues, sod it! Every one uses sierra hubs (nearly!) and there are no major complaints. Seems fine on mine, though that isnt saying much cos it isnt running yet.