Vowels 8 and 15, /ʊ/ versus /ɔɪ/    3 pairs      [bullboil.html]

Only three pairs emerged from the computer's search. There is no particular reason for there not to be more. It is a contrast between a short vowel and a diphthong with fairly close starting positions but a difference of lip-rounding. I have not heard of it being a problem to any learner.

The density figure is too small to be worth calculating.

bull boil 	
  bulls boils 
full foil 

John Higgins, Shaftesbury, December 2009