[ickystay]

A quick report. I bought a 600mhz motherboard to throw into a 400mhz iMac DV. I thought it would be a straight up trade. It's not. The processor of the 400 is taller and in a different location. That means that the heat sink needs to be moved and shimmed to be tall enough to come into contact with the thinner 600mhz processor.

Well, I used a scrap of 3mm aluminium for the shim, sandwiched it with the original heatsink (plenty of heat sink compound between layers), and put in the 600 board.

The computer is working. I worry that 3mm was not enough of a shim for a real tight squeeze between heat sink and processor. I downloaded an app called Temperature Monitor, but it says I have no sensors to monitor. Any symptoms I should watch out for related to over heating processor?

If anyone can post a better way to do this swap, I'd love to hear it. I have no problem going back inside this little bugger.

[BigBadBiologist]

Well, if you run some CPU intensive stuff (like a folding client or benchmark) and the computer doesn't lock up, then that will tell you that the CPU isn't overheating.

As long as there is good contact between the CPU, the shim, and the heatsink, it should be ok. I used to do this sort of thing when using laptop P4 CPUs in desktop boards. They required a spacer, so I lapped (sanded very flat) pre-1983 pennies to have a nice copper spacer that is about the same size as a CPU core.

When you say "plenty" of thermal compound, just make sure you don't use too much or it will actually inhibit optimal thermal transfer.

[Waragainstsleep]

You should check the heat coming out of the back occasionally too. This swap doesn't always work due to the extra strain on the PSU. The processor won't be the only thing running hotter in there.
If you are desparate for temp readings without installing your own sensors, temperature monitor should be able to read the sensor in your hard drive. Might give you some indication I guess.