WHAT DO ALL THOSE NUMBERS MEAN
We developed the most commonly used headset identification system – SHIS which stands for Standardized Headset Identification System. We use the measurements you get from the headtube and fork to influence the naming of different headset specifications.
The identifier starts off with two letters which signify the type of headset it is. “EC” stands for External Cup, and “ZS” stands for Zero Stack, “IS” Stands for Integrated System. The numbers following the two letters signify the headtube and fork dimensions.
Take this headset for example: ZS44/28.6 EC44/40. This headset would work for a press-fit headtube with an inside diameter of 44mm top and bottom (ZS44 upper cup and EC44 lower cup) , and the fork has a 1 ⅛ in (28.6mm upper steerer tube) to a 1.5 in tapered steerer (40mm crown race).
A ZS44/28.6 EC44/30 would work for the same 44mm press-fit headtube, but it has a 30mm crown race which would work for a straight 1 ⅛ in steerer tube. A ZS44/28.6 ZS44/30 would also work on the same 44mm press-fit headtube, and also work on a straight 1 ⅛ in steerer tube, but the lower headset assembly would not have as much stack height as the EC44/30 bottom assembly.
ZS44/28.6 EC49/40 will work on a press-fit headtube that has an upper inside diameter of 44mm and a lower headtube inside diameter of 49mm, and a fork that has a 1 ⅛ inch to 1.5 inch tapered steerer tube.
IS42/28.6 IS52/40 would work on an integrated headtube (IS) that has an upper headtube with a 41.8mm inside diameter (which we round up to 42mm) and a lower headtube inside diameter of 52mm, and the fork would have a 1 ⅛ in to a 1.5 in tapered steerer tube. IS41/28.6 IS41/30 would work on an integrated headtube that has an upper and lower headtube I.D. of 41mm and a fork with a straight 1 ⅛ in steerer tube.