Blue Smoke Squared
I admit I cannot take credit for thinking this one up, but after seeing the original by Flemming Kokio, I just couldn't resist. I decided to improve the design a bit and learn from his mistakes. I now have the fastest cube on the planet.
Step 1. Take your cube all apart (again). Remove the video card, modem and all the ram. Unscrew and remove the motherboard. Unclip and remove the processor, and pry off the copper heat spreader plate. It should pop off with little effort. Set the boards aside in a static safe place. Push the handle back in and remove all the screws holding it and the shiny bars around it. Should be 9 screws if I remember right. The handle will now drop out the bottom, and the large black heatsink should slide out the top. There are 2 screws which hold each linear slide to the heatsink.
Step 2. The biggest problem with this hack is what to do about the large inductor coil (marked in blue below) on the dual processor card. The inductor is an essential part of the regulator for the processors. The original coil was much smaller and mounted flat to the board. The new one will hit the large black heatsink. The inductor must either be moved, or room must be made for it. This is where my design takes a left turn from that of Flemming Kokio's. He simply moved the inductor. On my system I wasn't able to satisfactorily resolve the conflict between it and my ethernet card. If you move it more than 1" away, the system becomes unstable. Also, if you can, try to find a card that uses the 7410 G4 as it uses about 25% less power than the cards based on the 7400. The 7410 has a noticeably smaller die.
The red oval denotes a capacitor that should be watched carefully during reassembly. It can interfere with the heatsink. This will cause one of the CPU's to overheat.
Step 3. Remove the linear slides and disassemble the one under the area you wish to cut. It is necessary to cut away part of the metal guide rail, and almost all of one side of the plastic runner. This picture below shows how much material should be removed. Cut along the red lines. This plastic is brittle, so use a dremel tool, not wire cutters. This will cause the loss of the guide tabs on this side. That's OK though, the metal handle will hold it all together when you are done.
Step 4. Using a top end mill, I cut out enough of the heatsink to make room for the inductor. You can also see the end of the latch groove that I had to cut off.
Step 5. When putting the slide back together, take care to reinstall this shiny spring clip, shown in yellow. You will be unable to unlatch your cube's core without it.
Step 6. The copper heat spreader plate should be rotated 90 degrees so that the raised area contacts both CPU dies. This means drilling new holes in it. The image below shows the original holes circled in red, and the new holes circled in green. Make the new holes larger if you are as bad at locating hole centers as I am. You will also have to drill out the standoffs that aligned this plate with the old CPU card.
Step 7. Start putting the system back together in the reverse order of disassembly. Pay special attention to your cable routing. The picture below should help.
Step 8. I noticed that the heat spreader does not sit flat on my heatsink. It actually only touches about 25% of the area. My heatsink is warped a little bit. I compared this to another cube and saw the same thing. I tried to straighten it, but it just bent right back. So I filled that gap with thermal interface goop. I hate this stuff. It's nasty and messy. But it works real well and I had no choice. Only apply the goo after you are 100% certain everything fits together properly.
Step 9. Before final reassembly, put some thermal interface goop on each CPU die. Under normal circumstances, the goop should be applied as thinly as possible. It's only to fill the gaps in the surface. Actual metal on silicon contact has far better thermal conductivity. Here is a picture of the right amount to use.
Step 10. Reassembly. After step 9, attach the CPU card to its socket on the MLB making sure to include the barrel standoffs between them. You will not be able to use the clip that held the CPU to the heat spreader anymore.
Reattach the cables and screw everything back in place. Reinstall your ram and video card making certain to plug in the 3 cables that had to be disconnected from the video card area. Slide the case back on and fire it up. The Apple System Profiler will confirm that you are now running 2 CPU's. As will the CPU monitor in OS X. Have fun with your new toy.
The Fan Upgrade:
Every G4 cube has a bracket in its bottom for a 60X60x15mm fan. Every cube also has a 12V output on it's power supply board to drive that fan. This is an upgrade that every cube owner should perform. Even on single CPU systems, the difference in operational temperature in nothing short of amazing. It doesn't have to be a noisy or high powered fan either. Simply moving 10cfm will make all the difference in the world. If the fan is too noisy, install a resistor inline to slow it down a bit. This upgrade is MANDATORY for those installing a dual CPU card. Your system WILL overheat without it. Below is a picture showing where the fan should go.
This 12V connector, circled in red, should be used to power the fan.
Here is a closeup of the fan's label if you want to get this exact part. I can't hear my fan over the noise of the hard drive.
For people with hot video cards, I'd also suggest attaching one fo these nifty little fans to the end of the card. It's a squirrel cage fan (sort of) and blows to the side, instead of down. It is about as thick as the vent above the video card. This fan is noisy above 2-2.5V, so include a potentiometer with your installation.