I have always used Pyrex or Kimble-Kontes 16 X 125 screw top test tubes to form methyl esters of various compounds but lately I have had a horrible time keeping them from leaking. I've tried both manufacturers, other caps, foil, etc etc but about one in ten leak causing the BF3/MeOH to evaporate. My heating blocks have 16mm openings so I would like to stick to that size if possible. Are there any remedies out there?