I have the right type of long shank screwdriver to loosen the screws, but the plastics on these Macs are getting kind of old and brittle at this point and I don't want to break it using the wrong tool. What's best for opening them once the screws are out?

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There are "case crackers" out there but I've never seen one. I'm ashamed to say this but I use two wood chisels. The angled ends go together with the flat side of the chisels on the outside. With the tips in the groove, I then bring the two handles together and it pops them open. If the case is really tight I do this in various places a little at a time. Probably not the best idea but it works.