I checked my E39 - that has a hex (allen socket) in the top of the rod
which you use to stop it turning. To torque correctly you'll need one
which fits a torque wrench and a suitable spanner for the nut. Although
I'd guess there is a special tool. I'm pretty certain my long gone E28 had
the same arrangement - it's the only one I've changed the struts on.

Hello:)   I had this problem about 2 years ago on the passenger side and
took it in to the mechanic.  I remember him taking a large pair of
channel lock pliers to hold something under the fender while he
tightened the top strut nut.  My mechanic is now gone and I peaked under
the fender and dont see where could of put it to stop the strut from
spinning while tightening?  I dont want to put it on the chrome rod
unless that how its done? I put the car in the air and grabbed the tire
at 9 and 3 'clock and can see the whole assembly move.  Strut mounts
were new about 2 years ago.  I was hearing a noise over bumps on the
left side that why I went searching for the problem.  Thank :)Nate