Stack height designation for EC headsets includes 10mm cup (see tech specs). Any frame that uses an external cup headset with a 34mm upper head tube. inner diameter and 34mm, 44mm, or 49mm lower head tube inner diameter Uppers and lowers sold separately. Enduro stainless steel sealed bearings included on both the upper and lower. Upper headsets ship with a star nut which can only be used on forks with steel or aluminum steerer tubes. For forks with carbon steerer tubes you will need a compression plug (sold separately). (Never use a star nut on a carbon steerer tube). A crown race adapter (sold separately)will allow you to use forks with smaller 1-1/8" or 1-1/4" steerer tubes with a larger 1-1/2" bottom headset cup EC34 Lowers are not compatible with Surly 1-1/8" straight steerer forks