Lots of info here.
The impeller (properly called a "rotor") swells up from grease leaking out of gearcase. It should be a close fit on the eccentric and just touch the housing at some point on the outside. The other side should have a space "about" 1/8". As the shaft rotates that space progresses aroung then disappears at the outlet side. Then picks up another gulp of water as it reappears at the inlet side.
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