A late reply that may help others. OWC Aura Pro X2 is great.

2017 model, done in 2021

For anyone looking and wondering….

I used this guide to update my 2017 27” iMac. I’ve used a Crucial MX500 SSD for main storage and an OWC Aura Pro X2 for the NVME. I’ve gone from a best of 761 MB/s write, 1331 MB/s read to 2320 MB/s write, 2858 MB/s read with them configured as a Fusion drive.

Big things I noticed:

• READ THE COMMENTS UNDER EACH STAGE. There are useful updates for later iMacs.

• Watch out for the narrow cable from the logic board to mic in the chin of the machine. The ZIF is tiny but work - you just need a deft hand.

• Do take out the support bracket under the lower edge to the display (see comments).

• Well worth creating a bootable installation disk. https://support.apple.com/en-gb/HT201372 Creating a Fusion drive in the tools in Mojave and later are a breeze. https://support.apple.com/en-gb/HT207584

• As they are split 1TB / 480GB, I’m sure some would suggest splitting them, but this is more than fast enough to me to not have to think about what goes where.

This is MUCH easier to do if the support bracket along the bottom is removed first. See iMac Intel 21.5" Retina 4K Display (2017) Logic Board austauschen step 25.

Use a P00.

To replace it, introduce it like in the photo at the link to be able to hold the right most holes of the chassis and bracket. Replace the first screw, align the bracket to the horizontal and add the screws from right to left. Each one will bring up the next hole in the bracket close enough to screw through. Reverse those directions if you are a leftie.

This is MUCH easier to do if the support bracket along the bottom is removed first - more space to angle the board upwards. See iMac Intel 21.5" Retina 4K Display (2017) Logic Board austauschen step 25.

Use a P00.

To replace it, introduce it like in the photo at the link to be able to hold the right most holes of the chassis and bracket. Replace the first screw, align the bracket to the horizontal and add the screws from right to left. Each one will bring up the next hole in the bracket close enough to screw through. Reverse those directions if you are a leftie.

This is MUCH easier to do if the support bracket along the bottom is removed first. See iMac Intel 21.5" Retina 4K Display (2017) Logic Board austauschen step 25.

Use a P00.

To replace it, introduce it like in the photo at the link to be able to hold the right most holes of the chassis and bracket. Replace the first screw, align the bracket to the horizontal and add the screws from right to left. Each one will bring up the next hole in the bracket close enough to screw through. Reverse those directions if you are a leftie.

Agree. This is MUCH easier to do if the support bracket along the bottom is removed first. See iMac Intel 21.5" Retina 4K Display (2017) Logic Board austauschen step 25.

Use a P00.

To replace it, introduce it like in the photo at the link to be able to hold the right most holes of the chassis and bracket. Replace the first screw, align the bracket to the horizontal and add the screws from right to left. Each one will bring up the next hole in the bracket close enough to screw through. Reverse those directions if you are a leftie.

This is MUCH easier to do if the support bracket along the bottom is removed first. See iMac Intel 21.5" Retina 4K Display (2017) Logic Board austauschen step 25.

Use a P00.

To replace it, introduce it like in the photo at the link to be able to hold the right most holes of the chassis and bracket. Replace the first screw, align the bracket to the horizontal and add the screws from right to left. Each one will bring up the next hole in the bracket close enough to screw through. Reverse those directions if you are a leftie.

This is MUCH easier to do if the support bracket along the bottom is removed first. See iMac Intel 21.5" Retina 4K Display (2017) Logic Board austauschen step 25.

Use a P00.

To replace it, introduce it like in the photo at the link to be able to hold the right most holes of the chassis and bracket. Replace the first screw, align the bracket to the horizontal and add the screws from right to left. Each one will bring up the next hole in the bracket close enough to screw through. Reverse those directions if you are a leftie.

In 2017 too.

2017 here.

IMPORTANT - note Tanya’s comment. The narrow flat cable in the middle - it’s the mic cable on a *tiny* ZIF. See iMac Intel 21.5" Retina 4K Display (2017) Logic Board austauschen

Also has the extra bolt middle lower edge.

All T8 except the yellow T25.