The mounting of the rear fans on the antminer 17 and 19s was designed by someone who did not understand airflow. the fan blades are mounted directly against a perforated panel that cuts and disrupts the air flow; you can hear it in the high pitch gear whining sound these miners are known for, this is the sound of cavitation. Take the s9 line of antminer for example, it does not have this problem because the rear chassis housing panel is completely open the fan can do its job, When a fan is cavitating the air, this means it is not actually pulling the air... just like when a boat cavitates it stopes pushing, it is spinning the water and not pushing it, loosing suction. Just unbolt one of your fans and slowly pull it away from the miner you can HEAR the decline in noise and feel the fan start to pull against you. I really don't think the rear fans are doing anything but adding redundancy, which would be good in the case that one of the front fans stop working but this does nothing for cooling, I noticed the rear fans were much noisier than the front fans, this is because the rear are working in suction mode and the front are working as compressors fans, fans work much better with denser air, they have mass to grab onto, There is a vacuum Infront of a pulling fan, then you add in the turbulence of the rear chassis cover design you get cavitation, the fan is not able to do its job. When i built and installed these I could not believe the difference. Much quieter, 6 to 8% lower fan speeds, less power draw and lower temps, all four fans are spinning at much closer speeds with one another as well, cavitation allows the fan to Freespin because it is not picking up new air and has lost its ability to act as a pump, like pushing in on your  cars clutch, anybody with a boat will understand this well, I'm into race cars and engines, airflow is second nature to me. This listing is for a pair of spacers with hardware. your existing fan mounting screws  still work and new set of screws go from the inside into the back of the spacer block are supplied, no super long, hard to find, hardware is required. I can build through hole units if so desired, some guys find it easier to not unbolt the rear panel. I can supply hardware for this style as well, just ask...