PowerBaller

Ha i thought they were using silver copper alloy for such a higher price point.

The way how i use that pressure tester is:
1. Test the water block and radiator prior to the installation, in order to rule out faulty parts.
2. Before plugging the reservoir in to your loop, enclose your open loop with stop plugs after leaving one port open for the air pump, test the leak,
to ensure the critical part of you loop isn't leaking, ( the cpu block, vrm block, top rad, gpu block ) ;apply high pressure (.75 bar in the green zone).
3. Isolate the distro plate and test the plate itself with high pressure.
 
The leak test itself: if the air pressure did not drop a bit after 10 min, at the highest pressure in your green zone, it pass the leak test.
 
Now you can connect the reservoir. Even if it leaks. you have 95% confidence to tell that it is the junction between your pump and your distro plate is leaking, and the liquid wont drip on your electronic components, usually.

The silicon quality reference: usually, a 9900K requires less voltage to stabilize at a speed, than a 9700K. 
But specific cases are too discrete to be analytical, i am not surprise if a good 9700k can be driven to 5.1Ghz with 1.28 volt, while a terrible 9900K requires 1.33 volt to stay calm at 5.0Ghz.
There is no way i can tell you about the exact voltage setting; you have to find it empirically.  
But this is for certain: the steady state voltage is at least 0.03 volt above your min boot voltage.
As for what goal you should be choosing, if your ventilation is good enough, your cooler is capable of keeping you cpu below safe temp threshold, when your clock speed did not exceed 5.1Ghz. Start from 4.8 all core

The overall silicon quality of 9900K is better than that of 9700K. Even though you are still getting an average quality 9900K, the voltage require to stablized it are lower by 0.02~0.05V.
But just FYI, an air cooler is not able to suppress an OC 9900K, unless your ventilation and thermal interface are really well engineered, or you have disable Hyper Threading. 
Besides cooling, stable a 9900K requires at least above average power delivery you motherboard can offer.  And if you wish to overclock your b die ram too, avoid gigabyte motherboards. 
 
 
 
 

According to https://siliconlottery.com/pages/statistics. I assum an average 9700K is able to achieve stability at 5.1Ghz non avx, 4.9 Ghz avx, using the normal distribution.
 
The 9700K can turbo to all core 4.6Ghz at stock setting, first find the minimum voltage to boot it at all core 4.7Ghz, with 0 avx offset. ( Remove the limits, sync all mulitpiler to 47, manually override your core voltage), dont tune your RAM yet. Usually i start with 1.22 V for a 9700K, lower for 9900K and even lower for 9900KS. If windows is sucessfully booted, decrease the voltage by 0.01  until it cant boot. If it cant boot from the beginning, increase it by 0.02 and fire another attempt . Remark the minimum "boot" voltage for 4.7Ghz.
 
Now, estimate the min voltage of your goal by the following:  estimated voltage = ( min boot voltage /  4.7 Ghz) * Your Goal in Ghz, always round it up. 
i.g, my min boot voltage (4.7Ghz) is 1.20volt, i will start finding the min boot volt (5.0Ghz) at (1.20 / 4.7) *5.0 = 1.28 volt.
 
Once you remark the min boot voltage for your goal frequency, increase it by 0.03V, run the Intel Processor Diagonostic Tool first, then run some moderate stress test in Intel Extreme Tuning Utility, FPU in Aida64 for at least 15 min to detect if it result to Blue screen of Death. This usually detects severe instability. Increase the voltage by 0.01 if fail. 
You can also try finding the min voltage that does not result in instant failure. 
 
In order to emulate the most brutal condition your CPU will face, you need to test whether it can handle the AVX in Prime 95. Increase this voltage by a further 0.3, and run the small FTTS in Prime 95 for at least 20 min. If less than 2 of the workers ( thread ) fail, your CPU is stable in most situation at this setting. If non of them fail at all, your CPU is very close to flawless.
 
Stop at 1.42V unless liquid nitrogen is involved.
 
For long daily use, i wont't let the CPU operate at the voltage more than 1.35V; or at the setting that will boil the chip beyond 90 degree during non prime 95 stress test.
 
Dont even think about modifying the BCLK frequency if you are a beginner. It will directly effect the frequency of ram and pci devices. It is a pro touch. 
 
You will need the luck of getting a top 9% and top 1% silicon quality 9700K to acheive AVX stability at 5.0Ghz and 5.1Ghz. 

If hyperthreading is disabled, 9900K(S) is even cooler than 9700K at same freq, as they are made from better quality silicon in general, lower voltage is required to stable at a frequency. 
HyperThreading squeeze more potential from a physical core; such capability only requires a moderate amount of transistor in addition, at least not as much as that die need to be noticeably larger. When disable HT, the idle transistor however adds to the thermal area effectively. 
 
The setting really varys depending on the silicn quality of a CPU, which can varies greatly among a single model.  Even in the 9700K vs  9900K case, a ultra rare good 9700K stable at all core 5.1 at 1.265V (pass small FTTS in Prime95 without thread lost), while a trash 9900K is not even able to achieve that. 9900KS are officially binned by Intel, their quality are generaly excellent. 
 
Asus's ROG (not ROG STRIX) motherboard is able to predict your setting in AI Suite 3 (the max non avx stable, the max cache stable, and the max avx stable) , and even the silicon quality (in percentage) 

After disgusted by the smr drives from Seagate and WD (unless you are buying Seagate Exos or WD Gold / Black) . Toshiba X300/N300 become my only preference.
A 14TB helium filled N300, with 7200rpm and 256MB cache, while still being authentic PMR on amazon. What more can you ask when those spec converge into a signle model.
 
Toshiba is well known for being too "stubborn" to throw the horrific smr into application. Plus according to multiple harddrive annual faliure reports, Toshiba is always at the lowest spot. 

Gaming: 8C8T can already handels everything, even better than 8C16T. That means a 9900KS with hyperthreading off holds the absolute 1st seed, followed by HT off 9900K , then 9700K, then 9700. But even the actuall performance difference between 9700K and 9900KS is around 1%.
Productivity: The upgrade from 8C8T to 8C16T is far less than enough. I didnt see a reason not to swtich to AMD's hot trending 3950x 3900x, or the discounted threadripper. 
 
So unless you can sell your 9700 for a very good price, or you have other use for an extra CPU, upgrading 9700 to any 9th gen desktop cpu yields little significance.
 

Most AIO employs thin ratiators with low to decent fin density, which implies the pressure drop gradient is very small. Sandwiching a radiator (push and pull config) usually makes more sense in performance for thick radiators. 

Memtest86 is the way to go indeed. Run overnight 

Well i guess your suggestion solves. I reset to a convervatively stable setting first, OC the RAM to a degree to the ceiling of my expectation with tested stability, then start to tune the CPU. Now it survives a 5 min FPU at 1.35V 5.1Ghz, with peak avg tem lower than 70. Guess my chipest isn't that bad after all 

Module: Trident Z RGB 3200 CL14.  Dram Manufacure: Samsung, so certainly B die.
Run#1 Mem: 1.35 Volt, VCCIO :1.3 Volt, system agent voltage:1.3Volt, timing (17-17-34-2N) cant boot
Run#2 Mem: 1.45 Volt, VCCIO:1.3 , System agent Votage: 1.3 V, timing 17-17-34-Auto), stuck at the windows 10 icon screen.
Spec: 9700K with 137 setup parameter at stock, currently clocked at all core 5.0 Ghz. Mobo is Maximus XI hero, decent Power dlivery( with EVGA G3 850W)
 
has anybody ever stablized that before? how much voltage should i empower. 

AC68U is AC1900 standard. Its 2.4 Ghz band's is up to 600Mbps and 1300Mbps on the 5.0 Ghz band. 
 
While both AC86U and GT-AC2900 is AC2900 standard. The 2.4 Ghz band is however up to 750 Mbps and 5.0 Ghz band at 2167 Mbps, thanks to the additional internal antenna 
 
AC86U has a better processor, and the capacity of both the ram and the flash are double as ac68U. And most importantly, MU-Mimo (while AC68U only has Mimo ). These spec advantage greatly differentiate the performance while you have a lot of device connects at the same time. (i.e, you have a lot of files to sync at onedrive or google drive, while your wife / sisters / daughters are facetiming, and your brother is playing minecraft / netfilxing) .
 
GT-2900 is the ROG variant of GT-AC2900.
 
As for pricing. camelcamelcamle.com is a site you can track the price history of a specific item on amazon.com(or .ca). In Canada, the AC86U is CA$ 144 at blackfriday.