I would assume that the severe heat from the CRT, after years of use, caused this slight fracture in the case. There is no other physical damage to the case, besides some minor chips (ticks I call them) at the bottom, which I intend on filling, and if it had been dropped, the CRT may have shattered. The design does not look stable enough to tolerate a shock if dropped, and live to tell-the-tale, if you know what I mean.

Click to expand...

Now that I know what I'm looking at... yes, I think that probably is a thermally-related crack caused by differential expansion of the front bezel and the clear bucket. I would swear I've seen similar cracks on the inside of other iMacs... and those 17" CRT Studio Displays Apple used to sell that had similar cases. That being the case I wouldn't bother trying to fix it. It would take some pretty serious disassembly to replace the part, and as for gluing it would undoubtedly just open up again, since by now the plastic has probably shrunk a bit. (You could try filling it with a color-matching epoxy without trying to squeeze the crack shut, but it might just open up a new crack somewhere else as time goes on.). It probably won't fail "catastrophically" if you just leave it alone. Worst case it might spread until it reaches the front of the bezel and becomes visible from the front, at which point the case might exhibit some signs of compromised structural integrity. My guess is it will stop before it gets to any joints, however.

To be brutally honest... don't expect your iMac G3 to last forever. All CRT iMacs have a problem with the monitor failing eventually because of a combination of brutal thermal conditions and so-so parts quality. (The original tray-loaders have a fan, yet they fail at least as often as the fanless slot-load units.) These units were simply not designed to last.