so sierra right hand (as per haynes, surely they mean offside) front upright has a left hand thread, which makes sense, as the wheels being driven forward could only apply torque to the nut the tighten it not vice versa, however it appears the rear hubs are the other way round with the left hand thread on the n/s...

is there a good reason for this?

On a sierra the front wheels are being pushed forwards but the rear wheels are pushing the car forwards, the forces are opposite. I assume that ford have got it the right way round but I haven't worked it out for myself.

Duncan

ok... my tiny mind needs time to process that...