So 2 weeks ago I replaced a broken rear strut mount. The installation was made complicated because the new mount's mounting holes didn't quite perfectly match up with the holes in the top of the tower. I believe the one screw went in cross threaded. After about a 100km drive it started banging and squeaking again, the screws had loosened out. I took it back apart and rectified this problem my loosening off the other 2 screws and getting the suspicious one in first then the other ones. It seemed all good and quiet after that.

Now about 1000km later, its showing all the symptoms that the mount is broken altogether. It bangs and when you look at the top of the tower you can see the threaded portion of the top of the strut move up and down as you bounce the car. The screws are still in tight. How could the mount break this quickly? Being in there loose for 100km might have been enough to cause it? Or was there some part of the installation I could have done incorrectly to cause such quick breakage? Thanks.

I'll just add on here that the mount is defiantly broken. FML.

While I'm on here I have a question about the strut itself. Should it be able to pull in and out easily by hand? That strut did when I had it out, though the car doesn't bounce or show obvious signs of bad shock absorption.

Quite possible you got a defective mount factory made a crappy one. It happens Ive had bought a cheap set they lasted 4 yrs then wore out. I just replaced my rear struts last year the mounts were still good, Then I start to heard a thud sound and squeaks I replaced them problem solved. Also you might want to put a little amount of thread lock on the nut on the mount inside the trunk.