The Park Tool website has a page describing the standardized headset identification system.

I would like to confirm that I've understood it properly. Is this bottom section of a press-fit headset shown in the picture a ZS56/40?

NOTE: I've taken measurements with inexpensive digital calipers and the fractional component of the value can change significantly depending on how much force is applied when turning the thumbwheel.

lower press-fit headset section

1 Answer 1


Yes ZS56/40 - Zero stack with 56 mm bore diameter and 40 mm fork crown race.

It is zero stack because it is a black cup containing the silver bearing, but the bearing ends up inside the frame tube (i.e. not external). The 56.10 mm dimension gives a snug interference fit with a 56 mm head tube ( bore diameter). The 39.95 or so is about the 40 mm dimension that interfaces with the crown race/steerer tube. That last measurement seems slightly tight but possibly measurement error, tolerances aside it would be called a 40 mm.

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