1998 m3 coolant leaking after a bunch of changes

I have a question, I just replaced the water pump, fan clutch, all the belts, all the hoses, thermostat, and housing (water outlet). After a week or so, driving with the air conditioning on, when car turned off, there has been a hissing sound at the top of the water outlet and some coolant is coming out of what looks like the upper bolt on the outlet. The bolt is super tight. Also on the resevoir tank, the sensor is broken. Would this cause fluid to leak out, because I have to check it everyday day and fill resevoir. I put the chrome water outlet on, over the plastic one from factory. Any ideas. Also looking with a mirror yesterday, looking from bottom looking up on the water outlet, in two areas you can see a little part of the gasket. Would that cause the leaking, it looks flush and tight? Thanks for help. Is there a diagram of the cooling system, water outlet etc

also should i go back to the plastic housing, i have read on here that the aluminum ones aren't flush which could be causing to leak,

should the thermastat have a o ring or gasket?

The thermostat does have a O ring that has a split down the middle which it fits in and then yes there is an overall O ring which seals the thermostat housing to the head.
It sounds like you are missing two O rings.
There have been reports that the aluminum housings becoming porous after a while but I have not experienced that. The ABS plastic housing does become brittle and will fail.

O-ring should come on the thermostat, The other part is a profile gasket for the housing.

http://bmwfans.info/parts/catalog/E3...mp_thermostat/ see parts 6 and 8. Part 7 is not used.

The bolts don't need to be that tight. You really are risking them snapping off trying to grunt them down.