It stays in well. I've done it. Many bushings come with a wax like coating over the rubber to protect it. You'll need to use a dremel or something on low to just scuff up the rubber (inside the void) a bit, then wash the void out with some alcohol. Fill the void wait 8 - 12hrs or whatever the working time is, then install.
In the case of the e30 rear carrier mounts: The urethane in the void would form a solid 3" long chunk, the carrier mount bottom plate would prevent that from coming out even if it did come loose of the rubber.
The urethane stuff I bought is awesome, hardens up and feels like a hockey puck. Oil/gas resistant, I think it was nearly 20$ at Carquest.
Performance one step at a time.
Last edited by InfiniteDice; 11-07-2012 at 10:23 AM.