Just be aware that MANY 68040 chips on eBay have been remarked and are not what they are supposed to be.
The first letter (X or M) refers to whether the part was fully qualified or not. When Motorola was working on the 68040 series, they had bugs and glitches (called errata) that results in them not being “fully qualified”. Those earlier batches had XC as their prefix. The MC are fully qualified parts, which shipped basically after Apple stopped using 68040 CPUs, so nearly all CPUs in Macintosh computers are the XC part.
Then there is the 68040 part, you want the ones without any letters in that section.
RC or HRC are the remaining letters you’re looking for. HRC are supposed to be made to tolerate higher temperatures.
Pay attention to the mask information on the top right when shopping.
If you’re looking on eBay, Eric Woo seems to be one of the only legit sellers of 68040 CPUs.
If you’re getting a new CPU, you could consider a 33mhz or 40mhz part but to be honest, in the Quadra 605, especially if you’re going to run 25mhz, that’s all you’ll likely need. The risk of getting a remarked chip increases at 33 or 40mhz (these are more commonly counterfeited).
You can put a 40mhz chip in the Q605, but it will only run at the speed dictated by the computer, 25mhz. To run any faster (33mhz is commonly the only overclock available) you have to do some soldering.
-- so just wanting to know if there was any point to buying an '040 with a higher clock speed than 25mhz. I wanted to play around with Strata and a few other applications, and realized I needed an FPU to do so.
