Just did mine (1994 855M) and I had a heck of a time getting the T40 socket bit to line up in the rear bolt. If your bit is anything like mine, the hip will interfere with the valve cover and prevent a good straight insertion of the torx head. Since I did not have a long thin screw=driver type torx40, but rather a fat 3/8 drive socket bit, I had to improvise a bit by grinding down the end of the torx bit so that I got more bite. I strongly suggest a good soak of liquid wrench ahead of time as well. Of course, mine has been in for 205,000 miles, so it may be tighter than yours. BTW-I changed mine because all of a sudden the temp needle would hardly get out of the cool zone. The new -stat rides a little above center line. I find this a bit disconcerting as I was used to a lower reading. Now I am paranoid that I am gong to overheat. It actually never moves higher off the mark, but......

Once the 2 bolts are out, it's a simple swap out. Just remember to put the new rubber gasket around the new thermostat first, before removing the old one. You'll lose less fluid if you're lazy like me and did not lower the coolant level. The local cats seem to like the antifreeze I left out for them (just kidding)