You don't need the aluminum thermostat housing. You can put a few wraps of teflon tape around the o-ring on the sensor that goes in the lower hose and it will be good forever. As far as the expansion tank, the OE one was a problem and I don't think BMW improved them, so a good after market one will be as good or better than OE in this case unless someone knows different ? Same with the trans thermostat and coolant sensor, I've never heard anything about the ones that come with a good aftermarket expansion tank give problems ??
I have a thread about the second Standard Automotive tank that I put on my wifes E83 (same tank but thread might be in E83 forum) that was missing the groove for the lower retaining ring. If you look at how that is all put together, the upper hose holds everything together. There needs to be some movement somewhere because the radiator is going to expand at a different rate than the expansion tank and if everything is solid there will a lot of force put on the lower plate. IF there is enough movement in that lower tank connection even with the retaining ring then everything is fine but I don't think there is because people always have trouble pushing that connection together, never checked it myself though. Just saying, interesting
