Mammoth Uncensored Word List:
24 words ending with er, 16 letters long

autolithographer autoradiographer barocyclonometer biostratigrapher crystallographer greatgrandfather greatgrandmother intellectualizer lighthousekeeper microcalorimeter microseismometer neurotransmitter numismatographer photographometer photointerpreter photopolarimeter photosynthesiser photosynthesizer psychobiographer radiobroadcaster radiotelegrapher sphygmomanometer ultrasonographer videoconferencer