Well this exterior certainly isn't giving much away, but you can't always judge a book by the aluminum-and-glass shell of the iMac that you're reading it on. Let's start with what we know:

3.0 GHz quad-core Intel Core i5 (Turbo Boost up to 3.5 GHz)

8 GB of 2400 MHz DDR4 memory

Radeon Pro 555 GPU with 2 GB of VRAM

1 TB (5400-rpm) hard drive

802.11ac Wi-Fi and Bluetooth 4.2

43% brighter display (500 nits) with 4096 × 2304 resolution and P3 wide color gamut

A peek at the back reveals a bevy of ports. First, the usual suspects: a headphone jack, SD card reader, four USB ports, and an ethernet jack.

The un-usual suspects? This iMac's sporting two Thunderbolt 3 ports. These support double the bandwidth of the previous generation—each of these ports can push data at 40 Gbps.

That's enough throughput to drive four 4K external monitors—although with this hardware configuration, Apple says we're limited to two.

How are we supposed to get by with a total of just three 4K displays? How?

Confirming our suspicion that this is mostly a 2015 iMac with a facelift, this unit sports the same model number (A1418), but with a new EMC: 3069.

We grab our pizza cutter iMac Opening Wheel and go to town. Then, we bring it back home and use it to split open our fancy new iMac.

'Round and 'round we go, and the adhesive securing the display perimeter goes kaput.

When Apple introduced this form factor in 2012, replacing the much-loved magnets, we were sad and confused. Since then we've learned its secrets, and now we're on board with how easy this adhesive is to slice through.

After a familiar opening procedure, we get our first peek inside.

With all the glass and pixels out of the way, we immediately pick out some subtle updates.

First, the logic board has grown—encroaching on territory formerly held down by the right speaker, and even expanding towards the fan a bit.

Lastly, near the center of the bottom display bezel, just beneath the glass, there's a microphone! Whose ribbon cable routes right over the top of a screw, which is pretty weird.

Well, this part of the refresh is less refreshing than we'd like—the right speaker is no longer immediately removable. It's trapped pretty solidly by the new logic board contours.

No big deal; it just means your favorite set of iMac repair guides won't fully apply to this model. We'll get to work on that for you.

The rest of this iMac comes apart pretty much the way we expect, and we dispense with the power supply, hard drive, and fan without fuss.

And with a bit more elbow grease, we shimmy the logic board out of its slot to reveal ... removable RAM? We can hardly believe our eyes.

Yep, those are SO-DIMMs. Two of them.

Slightly less excitingly, there's also an exciting split heat sink. But seriously, look at that RAM!

It may not be as accessible as the (dead simple) RAM hatch found in the 27" iMacs—but still, this is a major win for upgradability over all the 21.5" iMacs with soldered RAM that we've encountered in recent years.

Before moving on, we take a moment to scope out the silicon these memory modules have to offer:

SK Hynix H5AN8G6NAFR 8 Gb DDR4 SDRAM (4 × 8 Gb = 4 GB per DIMM, 8 GB total)

This new heat sink design has us intrigued. What's hiding under there?

Warranty voiding stickers on the heat sink screws? That's odd. Could that mean...

Yes! The CPU is modular, too! It lifts right off with the heat sink, revealing a standard LGA 1151 CPU socket.

This CPU is quite well thermally-pasted into its heat sink; prying it out was surprisingly tough. Someone wants this processor to keep cool.

Finally, we're face-to-face with the star of this teardown: an Intel SR32W Core i5-7400 Kaby Lake CPU, 6M Cache, up to 3.50 GHz.

But with Apple's clout and famous negotiating skills, you'd think they could get a soldered CPU if they wanted. Plus there's that mysterious modular RAM...

Have you been hearing our pleas, Apple?

The final bit of interesting modularity on this board: a CMOS battery.

With all this new modular hardware, it almost looks like they ran out of space for this guy. It's hilariously placed vertically in this cute little battery toaster slot.

Or toast in your iMac for that matter...

Now that it's stripped of all accoutrements, we can ID this logic board's silicon:

Intel LGA 1151 CPU socket

AMD Radeon Pro 555 GPU

SK hynix H5GC4H24AJR-ROC 4 Gb GDDR5 2400 MHz SDRAM (4 Gb × 4 for a total of 2 GB VRAM)

Broadcom BCM5776 Gigabit Ethernet Controller (as seen in the 2015 iMac revisions)

Texas Instruments CD3215C00 (also seen recently in the 15" MacBook Pro)

Intel SR2C9 platform controller hub

Flip the board for more chips? Don't mind if we do:

Vimicro VC0359 camera processor

Cirrus Logic CS42L83 audio controller

MXIC MX25L6473E 64 MB serial flash memory.

Intersil 95828 HRTZ X707DMB

NXP 6142E

Intersil ISL6277A multiphase PWM regulator

Fairchild FDH10CJ

Turning our attention back to the display: the fancy new panel comes courtesy of LG (who also made the fancy old panel).

And that famously W-Fi allergic external monitor...

Now moving to the display chips, read on!

Take a peek at what powers those popping pixels:

Texas Instruments SN74LVC8T245 8-bit dual-supply bus transceiver

Texas Instruments BUF16821 Programmable Gamma-Voltage Generator and Vcom Calibrator

Parade Technologies DP665 LCD Timing Controller (the same found in the last generation iMac 4K)

Texas Instruments TPS54218 4.5 V to 17 V Input, 3 A synchronous step-down SWIFT converter

Texas Instruments TPS54320 4.5 V to 17 V Input, 3 A synchronous Step-Down SWIFT converter

Texas Instruments TPS65168 High Resolution, Fully Programmable LCD Bias

Previous iMacs featured a dual-mic setup, with two microphones hiding behind the front-facing camera. Two microphones allows the device to filter out ambient noise and produce a cleaner signal.

This year, it seems Apple switched to a single microphone—and moved it to the bottom of the display, behind the glass.

And that's that!

iMac Intel 21.5" Retina 4K Display 2017 Repairability Score: 3 out of 10 (10 is easiest to repair)

The CPU and RAM—two of the components you are most likely to upgrade at some point—are both modular.

The standard 2.5" SATA hard drive is fully upgradable—though you can't add a blade SSD thanks to an empty pad on the logic board.

Cutting the tape to open the iMac isn't too hard (with the right tools), but it must then be replaced to complete any repair.

Most replaceable components (like the RAM) are buried behind the logic board, meaning you'll have to take apart most of the iMac just to gain access to them.

The glass and Retina Display are fused together, increasing the cost of replacement.