Headset Size Calculator — Find Your Headset Type

Measure the internal diameter at the top and bottom of the head tube. Also measure whether it is threaded (external cups screw on) or smooth bore (internal/zero-stack cups press in).

A tapered head tube has a 56 mm bore at the bottom and a 44 mm bore at the top, matching a tapered steerer fork that flares from 1⅛" (28.6 mm) up to 1½" (38.1 mm). This provides more stiffness for braking and steering loads. Remember the SHIS numbers (44/56) are the head-tube BORE — the cartridge bearings that fit are smaller: 41.8 mm OD (ZS44, upper) and 52 mm OD (ZS56, lower).

The "56" in ZS56 (or the "52" in IS52) refers to the seat size, not the bearing. The cartridge bearing that fits a 56 mm zero-stack bore — or a ~52 mm integrated IS52 seat — has a 52 mm outer diameter. Likewise ZS44 takes a 41.8 mm bearing, IS42 (integrated) takes a 41.8 mm bearing (~42 mm seat), and IS41 takes a 41.0 mm bearing (~41.1 mm seat). Always order by bearing OD × inner diameter × angle (e.g. 52 × 40 × 45°×45° for the lower), not by the number in the standard name.

ZS (Zero Stack) has cups that press into the head tube. IS (Integrated) uses bearings that sit directly into the frame with no separate cups. EC (External Cup) has cups that sit on top of the head tube.

Yes, with a reducer crown race. A 44/56mm tapered frame can use a straight 1⅛" fork with a crown race adapter that fills the larger lower bearing space.

Service headset bearings every 6-12 months: disassemble, clean, re-grease, and replace bearings if rough or pitted. A well-maintained headset lasts many years.