Glad to give you some thoughts on the matter.
1) LAPPING
2) INSTALLATION
3) OPTIMAL CHOICE AND USE OF FAN
LAPPING AND "CUSTOM-LAPPED" VERSUS "FACTORY-CONDITION"
If you bought the "custom-lapped" version of the cooler, I won't recommend lapping it. If you didn't buy it "custom-lapped," I mildly suggest that you lap the base down carefully to just remove the convex-ridge that runs across it. But it is NOT ESSENTIAL THAT YOU DO THIS. Let me explain.
You will get slightly better performance for the U-120-Extreme if the surface is perfectly flat. The convex bow in the surface was, however, deliberately put there by ThermalRight's engineers -- it was a deliberate decision.
ThermalRight recommends against lapping, and the reasoning follows here. You should note from the available information sources that the copper heatsink base and heatpipes are nickel-plated. If you look on the web for "galvanic table," you will find that copper and nickel are right next to each other on that table. Using them as chosen by ThermalRight reduces the risk of corrosion around the solder-joints holding the heatpipes to the base. I say "risk," because that's all it is, and you should recall what old pennies look like, for being "old" and being handled much through their circulation history.
Galvanic corrosion is less likely to occur without immersion in an electrolyte, and this also speaks to air humidity and air salt-content. ThermalRight's "warranty limitation" per the lapping assumes the POSSIBILITY that grinding away one surface's coating of nickel may expose the unit to gradual corrosion which might cause the nickel plate to peel away.
However, SVC -- who provides the custom-lapping for an extra $30 -- is willing to assume warranty responsibility for their work, and assures that their process for lapping will not contribute significantly to any separation of the two metals.
IF YOU CHOOSE TO LAP, I recommend building a small wooden jig to hold the heatsink level until a flat surface begins to grow where the ridge-line of convexity had been. You can imagine how lapping a surface that is not already flat is more difficult than lapping a surface which is flat.
The gains you get from lapping, if the unit is properly installed in either case, are marginal to the actual performance of the unit unless you are looking for cumulative gains through several DIY enhancements (ducting, synthetic-diamond thermal paste, etc.)
INSTALLATION
The Ultra-120-Extreme should fit most LGA775 motherboards. Even so, in comparison to the original Ultra-120, it crowds in more heatpipes in order to make it a more effective cooler. And some LGA775 boards now come equipped with a "heatpipe necklace" covering the Mosfets around the CPU socket (hence, "necklace") and the Northbridge and Southbridge chipsets. This can leave only a very little clearance between the ThermalRight heatpipes and the copper-fins of the integrated heatsinks of the "necklace."
Therefore, as you would do for a custom-tailored suit, do some preliminary "fittings" to get a feel for how the unit will sit flat on the processor heatspreader.
This should also have you lining up the upper bracket with the mounting holes during such a "fitting."
I suggest the use of a magnetic phillips-head screwdriver during the installation process, so long as the screwdriver never comes within a few inches of any installed hard disks.
Per the INSTALLATION OF THE BRACKET UNDER THE MOTHERBOARD: FOR THE INITIAL INSTALLATION OF THE UNIT, YOU WILL NEED TO HAVE THE MOTHERBOARD UNINSTALLED FROM THE CASE, resting on either the plastic form or anti-static envelope in which it was packaged.
Grab the motherboard with fingers on the edge while gripping a PCI or PCI-E slot with the thumb, and hold it upside-down. Insert the bracket so the threaded fittings go through the four holes. Put the anti-static wrap on top (the exposed bottom-side of the bracket, so that when you flip the board and lay it back down on the table, the anti-static wrap will enable you to keep the bracket in its holes without touching the motherboard surfaces.
Now, follow the directions for applying and spreading your choice of heatsink thermal [paste] or [grease.] I recommend IC Diamond, but Arctic Silver 5 and JetArt CK4800 are easier to spread.
Carefully place the heatsink on top of the heatspreader cap so that it is oriented according to your intentions derived from the earlier "fitting" -- the objective having been to make sure that it clears obstructions and sits flat against the heatspreader cap.
By now, you should also have "fitted" the scissors-bracket so that it is already on top of the heatsink-base, with the "scissors" spread to match the holes (and threaded protrusions from the X-bracket you've placed on the motherboard underside.)
Initially, get one screw threaded about two turns in its socket. Hold the heatsink steady (to the desired orientation as determined from your "rehearsal fitting"), and slip an index finger or thumb under the heatsink fins to seat the scissors-bracket dimple into the depression on top of the heatsink base. Hold down the scissors bracket firmly with the finger or thumb. Now, preferably with the magnetic screw-driver (but if necessary with thumb and forefinger) -- carefully place a second spring-loaded screw in the scissor-bracket hole diagonally opposite to the one which has just been secured, and carefully get the screw started in the threads, twisting it, too, just a couple turns.
Insert the remaining spring-loaded screws into the yet-unsecured scissor-bracket holes and get them started in the same manner.
Go from one screw to the next -- tightening a couple turns on each screw, until all the screws are fully tightened and will not turn further. You do not need to exert additional force on these screws -- just make sure they are no longer loosely threaded and that they are threaded as far as they can go.
If you found that there are very narrow clearances between other motherboard components and the U-120-Extreme's heatpipes, I advise against "twisting" the unit much to better spread the thermal interface material -- unless, of course, you have an idea from your "initial fitting" as to how much the heatsink can be twisted either way before encountering an obstruction.
REMOVAL AND REINSTALLATION
It is a myth that the entire motherboard needs to be removed in order to uninstall and reinstall the TR-Ultra-120-Extreme (or "original").
If the motherboard uses 1/4" (6mm) brass or aluminum standoffs, the threaded X-bracket extrusions should remain seated in their holes upon reversal of the above installation procedure. If the motherboard uses 3/8" (8mm) standoffs, you should be able to temporarily shim the X-bracket under the motherboard by carefully inserting a tapered ALL-PLASTIC dime-store ruler. I have found that popsicle sticks also serve the purpose, but just make sure that whatever you slip under the motherboard can be easily retrieved from it later.
CHOICE AND USE OF A FAN
There are various priorities and preferences people apply to their fan choices. Some use Scythe fans which just push about 60+ CFM of air through the heatsink fins at the fan's top-end speed. I like to use a "beefy" fan with a strong motor capable of pushing in excess of 105 CFM with a high exhaust pressure rating in the fan-specs. Nobody likes noisy fans, but a high-output fan may have a precision motor with no motor-whine, yet running the fan at its top-end will generate "white-noise" from air-turbulence.
If you choose the latter approach, it should give you more flexibility in cooling, but you should be able to control the fan-speed automatically through thermal sensors -- either on the motherboard and using software like SpeedFan, the motherboard BIOS, or a software-driven USB fan-controller with thermal sensors. Stay away from the Sunbeam-Tech Theta-101 until after I've got the Federal Trade Commission to crawl all over them like maggots on a dead-bunny-rabbit, and they are compelled to throttle their sub-contractor by the neck until their product matches its advertised promises.
Enjoy the enhanced cooling, lower load temperatures and higher achievable clock-speeds with your ThermalRight Ultra-120-Extreme, and remember -- "On a hot and sultry night, your computer will be alright, as long as you use a THERMAL-RIGHT." [Save the Burma-Shave for your morning-after five-o-clock shadow.

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