One thing I found out with aftermarket motor-mounts is the twin holes in the mount for the small allen bolts may be improperly spaced and require elongating them slightly with a drill to prevent the possibility of cocking/cross-threading the bolts into the crossmember. Check them before attempting to tighten them with any force. Make sure the threads on the bolts and in the crossmember are clean. With the mount in place, you should be able to easily screw the allen bolts in by hand, or at least by hand on whatever allen tool you're using. Force should only be needed for the final tightening.

In my case, the holes in one of the German aftermarket replacement 240D mounts were slghtlty off, though they looked OK at a glance. I probably spent 2 extra hours struggling with it before I finally figured out why one allen bolt kept crossthreading. Fortunately, I was able to re-thread the crossmember from below, before slightly modifying the errant mount's holes to finally make it fit my car!

Happy Motoring, Mark

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DrDKW