The spelling of "dilater" may seem confusing, but it can be explained using IPA phonetic transcription. The correct spelling of this word is actually "dilator," pronounced /ˈdaɪlətər/. The "e" in the original spelling is unnecessary, as it does not reflect the sound of the word. Instead, the correct pronunciation is achieved by using the letter "o" to represent the schwa sound in between the "l" and "t." So, remember to spell it as "dilator" next time you encounter this word.
One who.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.