sounds like you may have purchased the wrong fan/cap combo.
most fan motors are what I refer to a "commissioned motors" built by a motor co. specifically for a particular customer i.e.- the maker of the a/c unit.
these motors typically have an odd spec or two that makes replacement a headache trying to find one w/o having to have a contractors license to order the oem motor from a hvac supply house.
if you have one around, take it to a Graingers outlet and they will hook you up with their specialist to help identify and procure the appropriate motor.
don't be surprised if they can't find an exact match for the same reason that I mentioned above. best of luck.