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Inside many dishwasher pump assemblies is a screw that goes through the upper impellor and screws directly into the motor shaft. Since the motor shaft and screw are made of metal...you guessed it...they can / will rust. The screw can rust and become very tight and difficult to remove!! These screws are normally a right ( regular ) hand thread, socket or nut driver can be used to remove the screw. One hard thing to do is to hold the impellor still so the screw can be removed, they often just turn together. The impellor may be jammed with a screwdriver to hold it still so the screw can be removed...sometimes the impellor can be help still by placing a rag on it and hold tight with your hand, the rag will help prevent a sharp edge on the impellor from cutting your fingers. The odd time that screw is too rusted into place, soaking a little rust buster or 3in1 oil on it over night may help loosen it. Sometimes trying to tighten the screw a little at first will help break it free. There are some tool ( see below ) that can be purchased to help hold the impellor still so the screw can be removed, these are only for a few models and pump assembly styles.
Once the screw is removed, the next obstacle is the impellor can also get stuck onto the motor shaft. Many times I have to break off the old impellor. A sharp blow straight down with a hammer and flat screwdriver will often crack the plastic impellor...careful not to damage the pump body! If the motor shaft has indeed rusted, clean it up really good with sand cloth.