Anything non-conductive would do. Just make sure the different metals that it is separating are not electrically connected in any way.
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even if you install this plastic piece, you will not prevent electrolysis. Try using a sacrificial anode at the bottom of the motor and chemical injection from the surface to the bottom of the motor to be mixed and carried by the produced fluid to surface. Yes I can send you photos of sever damage caused by electrolysis, send me you email.
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