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First I suggest you to read the other posts and specially the one from rtarkie. It is the method I use to clear a rad free of dirt and sand.
About the fan clutch, the original was probably a Aisin brand.
Many years ago I took apart one to see how it was made and operating. Some people assumed that viscosity was engaging it when warm. It made no sense to me and this is not true. In fact it is the opposite, warm oil is less viscous at high temperature.
What is engaging the clutch is the thermo-bimetal coil in front. It moves the center shaft just about 1/12th of a turn, say from 1 to 2 o'clock. That is enough to open the inside blade valve (that blocks two openings) and release the fluid that is acculated in the finned donut (you can see the shape if you look in front, it is around the thermo-coil). When fluid is released it floods the friction disk hense engaging the fan. Even when engaged the system slips a little. That motion pumps the fluid back in the donut. It will flow back in the friction disk area if the thermo-coil is hot, and that will keep the clutch engaged. If the rad sends cooler air at the front of the fan clutch the thermo-coil will close the blade valve and retain the oil in the donut and that dis-engages the clutch. But there is still friction enough to turn the fan. But it can be stopped by hand. DO NOT DO THIS WITHOUT WEARING HEAVY LEATHER GLOVES.
If you live in area where road salt is used in winter I would suggest you use heavy grease and fill/cover the thermo-coil. That will prevent corrosion of the thermo-coil and also seisure of the shaft is in the center of the thermo-coil. I have done this for more than 20 years with success. I am in Montreal region and winter is corrosive.
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