I've been thinking of getting the same CPU cooling solution as Pistol which is a Thermalright Mach Zero with a Noctua NF-A15. However I found three versions on the website that I went on. They are called "Thermalright HR-02 Macho Zero", "Thermalright HR-02 Macho Zero 120mm Fan Duct (100700727)", and, "Thermalright HR-02 Macho Zero 140mm Fan Duct (100700728)". Also the NF-A15 is a 140mm fan but the website says that it uses 120mm fan mount holes but I am not sure. So which version of the Macho Zero should I use?
The fan ducts are probably just that: fan ducts.
Thermalright has them to help (semi-passively) channel air through the heatsink and out the back of the the case. The 120mm duct is probably for cases with a 120mm fan exhaust, and ditto for cases with a 140mm exhaust.
Both Thermalright and Noctua do something similar with some of their fans in which they have 120mm mounting holes, feel free to mount any of your choosing.