How oil pumpjacks work.
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Previously:
https://www.homemadetools.net/forum/...269#post130848
https://www.homemadetools.net/forum/...967#post148950
https://www.homemadetools.net/forum/...482#post110702
https://www.homemadetools.net/forum/...173#post138380
https://www.homemadetools.net/forum/...881#post183579