You are correct The BMW casing is machined to fit the Caterham diff cage. A Caterham back plate is applied to clear the DeDion tube. The Caterham diff cage uses the same bolts as the Sierra diff, so fitting the diff isn't the problem. The drive shafts and prop on the other hand is another question. Drive shafts are available from CC, at about £300 per side. The prop is more difficult, because I am not sure if a Mazda gearbox to Sierra prop was ever created by CC. Maybe a 620S uses it, but all other cars with a Mazda box (introduced 2013) will have been connected to a BMW diff (introduced in 2010/11 ish). If its available then it will be in the region of £260(ish). Why are you looking to change to the Sierra diff, is it because of the threads about the Titan diff? A 420R 2016 car will have originally had a carbon plate Titan diff in a BMW case, which unfortunately seem to be expiring. If I had a 2016 420R I would perform a preload check on the LSD to see where I was on the service life. Others has posted Phils wisdom from RRT, 40ft/lb when new, 20ft/lb rebuild needed and 15ft/lb impending doom. You then have a choice in cost order 1, Rebuild with Sintered plates (as per CC post 2018), but this will make a lot more noise than your 2016 carbon plate version. 2, Replace the LSD with other brand, probably where I would go, I will be looking from a LSD with molybdenum plates like the Tracsport. It will be more expensive than a Titan rebuild. 3, Convert to another solution. As a side note, the Sierra diff wasn't without its faults either. Many converted from Sierra to BMW when it was first release because of the noise they used to generate, so do your research. You don't want to jump from the frying pan into the fire.