Thanks much for this guide. I just completed installation of an E5-2697v2 (the same 12-core CPU that Apple ships) that I purchased off eBay for $550. I found a few things that were not quite accurate and left notes above in the relevant steps. Basically, both places that claim that the screws are T7 were actually T8 in my machine (apparently assembled in mid-2017 due to the fact that it came with Sierra 10.12.6 installed). Also, as a previous comment noted, my machine also had tamper-evident screws holding the CPU heat sink bracket (step 25). The T10 driver makes short work of the stickers. By far the trickiest part of the reassembly was installing the CPU heat sink bracket. After starting screws on one side of it, it takes considerable force to bend it down enough to get the screws on the other side started. And it’s hard because at the same time you have to keep the CPU from falling out on the opposite side. Really not too hard. Just get a nice work area with plenty of light and keep track of your screws.
On my Mac Pro (assembled mid-2017) these screws are covered with a black sticker presumably to indicate tampering. If you did not know they were screws it would not be obvious. You have to just put the T10 driver right in the center and start turning; it quickly breaks through the sticker.
The trickiest part of reassembly was attaching the CPU heat sink bracket. Some instructions for that might help. Quite a lot of force on the bracket is needed to get the screws in, due to the spring-loaded nature of the bracket.
On my Mac Pro (assembled mid-2017) these are T8 screws. In fact, there were no T7 screws anywhere in my machine.