Assuming that the metal case jacket is not warped or bent, you need only lower it over the chassis with about a 2" space between it and the backplate, and then slide the jacket rearwards as you guide its rear edge into the recess in the backplate. What kind of trouble are you having?

Most of the (undeserved) bad reputation of the 9500/8500/8100 derives from the need for almost complete demolition before replacement of DIMMs or the battery, rather than from difficulty in putting the case jacket back on.

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