Is the CPU on the iMac G3 upgradable? How is it mounted? Are third-party processor upgrades available?

Unfortunately, the CPU on the iMac G3 is not mounted on a ZIF socket like the models in the Power Macintosh G3 series. The tray-loading iMac G3 models -- those with 233 MHz, 266 MHz, and 333 MHz processors -- have the processor soldered to a processor daughtercard, and the slot-loading iMac G3 models -- those with 350 MHz and faster processors -- have the processor soldered directly to the motherboard.

The processor daughtercard in the tray-loading iMac G3 models also contains the ROM. Consequently, the only feasible way to upgrade the processor in these models is to replace the original processor daughtercard with a processor daughtercard that contains both a faster processor and an original ROM. To upgrade the processor in the slot-loading models requires one to replace the entire motherboard with a motherboard equipped with a faster processor.

A number of processor upgrade card companies -- FastMac, Newer Technology, PowerLogix, and Sonnet -- released upgrade cards for the tray-loading iMac G3 models that essentially were original iMac G3 daughtercards with the original processor removed and a faster one soldered in its place. Sonnet, perhaps most interestingly, also added a handy Firewire "400" port to their upgrade cards.

Specs on these upgrade cards for the tray-loading iMac G3 models -- all long ago discontinued -- are listed on the "By Compatible System - iMac G3" page of the archived "Pre-Intel Upgrade Cards" section of EveryMac.com, but also are referenced below for your convenience:

Although it required swapping the entire motherboard, FastMac also offered an "upgrade card" for the slot-loading models with 350 MHz, 400 MHz, 450 MHz, and 500 MHz processors (except the "Early 2001" Flower Power/Blue Dalmation systems and presumably the later "Summer 2001" systems as well).

Given the other limitations of the tray-loading iMac G3 models, it does not make financial sense to invest much in a upgrade card. However, if you locate an extremely cheap iMac G3 upgrade card and you like to tinker with old Macs, you probably would enjoy the experience.

Alternately, a motherboard swap for the slot-loading models makes even less financial sense, but if you don't mind the risk of killing a slot-loading iMac G3 entirely, and you have a lot of hardware experience, overclocking it is an option.