A customer who lives off Newhaven Road in Bonnington dropped three large boxes on the bench a few months back and asked us to finish a build he had started on a Saturday morning. It was a strong parts list on paper — a current-generation CPU, a motherboard he had bought from one review and memory he had bought from another, a graphics card that was supposed to post the week’s best benchmarks, and a case he already owned from a previous build. By Sunday afternoon the machine would not post, the memory kit refused to run at its rated speed, and the graphics card was pressing against the front fan bracket. None of the parts were faulty. They just did not want to live together.
This is a recurring story at the Bonnington end of the bench, and it is almost always the same handful of mismatches. The retailer’s website lets you add anything to a basket, the reviewer who praised a part never used it in the case you own, and the compatibility checkers only catch the obvious failures. The notes below are the quiet mismatches we see most often on real builds coming through the Leith Walk and Newhaven side of the city, and the sanity checks that save a day of rebuilding.
CPU and Motherboard: the Socket and the Chipset Are Not the Same Thing
Fitting a CPU into a motherboard with the right socket is the easy half. The trickier half is the chipset — the controller family on the board that decides which generation of that same socket it supports. A current socket can host two or three CPU generations depending on BIOS, and a board that was manufactured a year before your chip was launched will usually need a BIOS flash before it posts. Some boards have a USB flashback button that handles this with no CPU installed; many do not, in which case the board will come up to a black screen and the only way out is a borrowed older chip or a trip to a shop. Always read the board’s own CPU support list for the exact revision of the chip you have, not a generic family name, and if the board was in stock before your CPU was announced, assume a flash is needed.
The second trap on the same page is memory support. The chipset decides how many sticks of a given speed will run at that speed, and the number is almost never four. On a board with four DIMM slots and an enthusiast CPU, two sticks will post at the rated kit speed, but four of the same sticks will often fall back to a slower JEDEC speed on boot. If a four-stick kit is what the build needs, buy a kit sold as four matched sticks, run the memory profile from BIOS, and expect to tune down one or two speed steps on anything above the board’s published four-stick rating. There is more on when it is worth starting fresh instead of forcing older parts into a new build in our note on reusing an older SSD, GPU and case in a new custom PC.
RAM: Two Sticks or Four, and the Profile That Does Not Load
The second most common bench surprise is a memory kit that boots, but at a speed that looks nothing like the number on the box. Modern DDR runs well below its rated speed by default — the fast speed only comes on when the memory profile (XMP on Intel platforms, EXPO on AMD) is turned on in BIOS, and even then the result depends on how many sticks are fitted, which DIMM slots they go in, and the memory controller silicon lottery for your specific CPU. A kit rated at a high speed with four sticks on paper can refuse to post at that speed on a specific chip, and no amount of restarting the installer fixes it.
The practical rule we use at the bench: for most home and gaming builds, pick a two-stick kit at a sensible speed over a four-stick kit at a flagship speed. Fit the sticks in the slots the motherboard manual tells you (almost always the second and fourth from the CPU on a four-slot board), turn on the memory profile in BIOS, and verify the speed in Windows task manager under Performance before celebrating. If the sticks are physically tall with heatspreaders, read the next section before you also buy a big air cooler, because the two parts often fight for the same airspace.
CPU Cooler and RAM: the Clearance Nobody Measures
A large dual-tower air cooler is still the quietest way to keep a hot CPU in check, but the front tower almost always overhangs the first DIMM slot. If the memory kit has tall decorative heatspreaders, the cooler either will not fit at all or will force the front fan to be raised, which breaks the airflow pattern the cooler was designed for. The datasheet number to compare is the memory height in millimetres against the cooler’s published RAM clearance; if the two are within a couple of millimetres, treat it as a mismatch unless you are happy raising the fan. The same conversation applies to a 360 mm all-in-one radiator in a case that only has top clearance for a slim 240 mm — the fans and the motherboard VRM heatsink end up occupying the same inch of space.
Graphics Cards: Length, Width and the Case Floor
Current flagship graphics cards have grown in all three dimensions, and the case specification sheet is where the compatibility conversation lives. The number that catches people out is not length but width — the number of expansion slots the card occupies. A card listed as three slots will often need the fourth slot clear for airflow, and in a mid-tower that puts a fan directly over the lowest intake. The second number is the length against the front fan bracket; our shelved-and-measured rule of thumb is to leave at least ten millimetres of air between the end of the card and any front-mounted fan. The third is sag — above a certain weight, the card needs a support bracket or it will slowly flex the PCIe slot over the first year.
If an existing case is being reused for a new build, measure the longest internal length with the front fans in place rather than trusting the case’s marketing number, and compare it to the card’s own listed length. Our walk-through on planning a first custom PC build covers the order in which to decide these numbers before anything is bought, which is almost always cheaper than returning parts the week after.
Power Supplies: Enough Watts, Wrong Cables
Modern graphics cards above a certain tier use a new high-power connector — the twelve-pin header officially known as 12V-2x6, which replaced the earlier 12VHPWR with a tightened safety specification. A power supply bought two years ago will almost certainly not have one of those cables in the box, and plugging the four or five older PCIe cables into the adapter that ships with the card works, but is far from the clean install people expect. Supplies carrying the newer ATX 3.1 label will usually have a native cable in the box; older ones often need a replacement modular cable from the manufacturer, which is specific to that power supply family and not interchangeable between brands. Watts rating is almost never the problem on a sensible build; cables and connectors usually are.
The other quiet mismatch is CPU power. A hot current CPU wants two eight-pin EPS cables feeding the top of the board, and a budget power supply often ships with one. The board will post with one connected and complain under load, which looks for all the world like a thermal problem until the second cable is fitted. If the build then needs the operating system set up fresh afterwards, that is handled under our OS installation service while the bench verifies the memory profile and the drivers in one pass.
M.2 SSDs and the Ports They Quietly Disable
Modern motherboards usually have two or three M.2 slots sharing PCIe lanes with the SATA controller or the chipset’s secondary network and USB ports. Fit a drive into the second M.2 slot on a mid-range board and the motherboard will often disable two of the SATA ports to free the lanes, which means a hard drive you planned to reuse suddenly will not detect. The motherboard manual lists which slot steals which ports; it is not consistent across brands and it is not consistent across chipsets in the same brand. For an upgrade to an existing build, the safest move is to fit the fastest drive in the primary M.2 slot and keep the SATA layout as it is; for a fresh build, decide the storage layout before the board is picked rather than after.
If the clash is only noticed after the OS is installed and the data from an old drive is now stranded on a port the board has disabled, the data is usually recoverable with the drive moved to a different slot or an external adapter — our data recovery service handles exactly this kind of job without touching the new build.
When It All Boots but Will Not Post
A build that goes together physically but shows a black screen on the first press is almost always one of three things: a BIOS older than the CPU, a memory profile the chip cannot drive, or a power connector not fully seated. The order to check them is the opposite of the order most people pick: push every power cable in again until it clicks, clear CMOS, boot with one stick of RAM in the manual’s preferred slot, then revisit the BIOS version. Our longer walk-through on a new custom PC that will not boot covers the sequence in more detail. Beyond those three, the fault is usually a specific board revision mismatch rather than a dead component.
Bring the Parts List In Before the Delivery Van
Almost every mismatch in this piece is cheaper to catch on a spreadsheet than in a box. If a Bonnington, Leith or Granton-Pilton customer is a week away from clicking buy, we are happy to look over the list — CPU, board, memory kit, cooler, case, graphics card, power supply, storage — and flag the clashes before anything is ordered. A short bench review beats a day of returns, and anything already bought that will not fit can be routed to our IT recycling and e-waste disposal rather than left in a drawer. If the build is coming to us from start to finish, the whole job sits under our custom PC builds service, including the parts matching, the first boot and the operating system install.
Collection runs cover Bonnington, Leith, Easter Road, Granton-Pilton and the Portobello side of the city on the regular north Edinburgh route, and we are a short drive from Newhaven Road to the EH11 workshop. The Bonnington custom-build page has the specifics, and if you would rather talk the list through first, book a bench review or drop us a message and we will tell you which parts are at risk of fighting for the same inch of space.
Last updated: 28 October 2026