The iMac "Core i5" 2.3 21.5-Inch Aluminum (Mid-2017/Kaby Lake) features a 14-nm "Kaby Lake" 2.3 GHz Intel "Core i5" processor (7360U) with two independent processor "cores" on a single chip, a 4 MB shared level 3 cache, 8 GB of 2133 MHz DDR4 SDRAM that is reportedly onboard -- ''but actually can be upgraded later'' -- a 1 TB (5400 RPM) hard drive, and an "integrated" Intel Iris Plus Graphics 640 graphics processor that shares system memory.
You can check a full run down of your systems specs at https://everymac.com/systems/apple/imac/specs/imac-core-i5-2.3-21-inch-aluminum-mid-2017-specs.html.
For the logic board replacement, check out the ifixit guide at [guide|103425].
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