The mounts that fit into the crossmember are held in by bending the edges(tabs) of the mount over the crossmember. They come out the same way...you have to either grind the tabs off, or straighten them out with a hammer/chisel, and then knock, or press the mount out of the crossmember.
If you are changing yours, I'd suggest you soak the outer circumference of the mount with penetrating oil, to make it a little easier to press it out.
Some are harder to get out/in than others, depending on how rusty it is...
The hardest part is actually just getting the crossmember off to start with!
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Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"