NOTE: This article was written before the 2001 G4 Towers were released. The 2001 G4s use a 133mhz system bus speed so the CPU modules are set for a Bus/CPU ratio that assumes a 133MHz bus speed. The pre-2001 G4/AGP systems use a 100MHz bus, so the Bus/CPU ratio of a 2001 G4 module (if it works - it may) would be running the CPU 33% below the rated speed. (I.E. Using a Dual G4/533 module in a pre-2001 G4 with 100mhz bus speed would be running the CPUs at 400MHz (assuming the module was pin compatible) instead of 533Mhz, since the module is set for a 4x ratio (4 times 133MHz bus speed = 533MHz CPU speed). Resistors on the CPU module set the bus/cpu ratio.

OS 9.1 Update Problems: [Update - As noted in the June 22, 2001 news page and in the FAQ, here, Sonnet has issued a warning about their dual G4 "Duet" CPU upgrade regarding a compatibility issue with revision 3 of the Uni-N motherboard chip. [Update - as of Dec. 2001, they noted a rev 7 or higher is required.] (Some machines may have later revs.) For those with problems running OS 9.1 with a swapped in Dual G4 module, I wonder if their Uni-N rev is earlier than revision 7. For how to check this using OS 9.1's Apple System Profiler, see Sonnet's Errata Note. (On 7/16/2001 a reader with a Dual G4 swap and problems in OS 9.1 noted his Uni-N was a rev 3. To check the revision using OS 9.1's Apple System Profiler with the Dual G4 module installed, he said he had to remove the Apple CPU Plugins extension for stability. He said "It is also very stable under 9.0.x (I use 9.0.4) including dual CPU work as long as I use Apple CPU Plugin 2.3 through 2.5. ASP even reports the second processor." ]
Two readers reported that OS 9.1 would not run reliably on the dual G4 CPU swapped systems. I suspect these both are pre-Gigabit models but just a FYI that there may be problems with OS 9.1 running on a G4 that's been updated with a dual G4 module. The problem systems had the 2.4 G4 firmware update applied, so not sure if that was a factor.

Update - Reader Tip for OS 9.1 with DP CPU Swaps: On 4/16/2001, a reader sent a tip that he said solved his freezes after updating to OS 9.1:

" Mike (and Jamie):
I told you I'd write back if I found a solution to this problem (upgraded G4/450 to dual G4/500 and OS9.1 would not run). I have. The solution is to downgrade "Apple CPU Plugins" from version 2.7 (and up) back to version 2.4.

I believe 2.4 came with my original G4 software (Oct. '99 G4/450 AGP).

I borrowed an G4 install disk from a friend who just received a new G4/733. This was the only disk that I could get to install when booting from the CD. It installed "Apple CPU Plugins" version 3.0 (which also caused freezes on my machine). After I downgraded to version 2.4 everything has become stable and works great.

Now I'm off to try and get OS X to install. I keep getting Kernel Panic errors on bootup...
-Brian"

Component Part Numbers: A reader posted info on the forums here some time back on the part numbers of the Dual G4/500 module and the dual CPU heatsink:

• Dual Processor G4/500 MHz CPU module: # 661-2325
• Heatsink: # 076-0812

He noted the dealer price (no markup) was $15 for the heatsink but $1170 (!) for the DP 500 CPU module. I've had readers say they bought them for much less. (Check Ebay.com, etc.)-Mike

This is what your single processor G4 looks like to start. First, remove the heatsink clips by pressing down on the heatsink gently, and use your fingers to pull the clip toward you gently ro release it from the processor card. once you have one side done, move to the other, the clips come off very easily.

Once you have removed the heatsink, remove the 3 screws that are holding the G4 module to the logic board. Then, gently, rock the processor card back and forth pulling it away from the logic board, as straight up as possible. The socket and pins are a LOT more fine than a ZIF module, care is needed here!

Once you have removed the G4 processor, you will see the socket that the processor attaches with.