Bike Headset Standards Explained: SHIS Codes, Sizes & Compatibility | BikeQuiver Cyclopedia

Also called: Aheadset, Conventional threadless, Standard 1-1/8

Head tube bore 33.9-34.0 mm; cups press in and sit outside the tube; 28.6 mm steerer; 30.0 mm crown race; SHIS EC34/28.6 | EC34/30

The threadless system that replaced threaded headsets in the 1990s. Cups press into a 34 mm bore head tube with the bearings sitting in visible cups above and below the tube. The stem clamps directly around a smooth 28.6 mm steerer, and preload is set with a star nut and top cap.

Why it exists: Dia-Compe's Aheadset design (early 1990s) eliminated threading, made forks easier to swap, allowed stiffer clamp-on stems, and simplified adjustment.

Typically found on: 1995-2010 road and mountain bikes, modern steel/titanium frames, touring and commuter bikes

Also called: Semi-integrated, Internal headset, Low profile, Perdido, Campy-style ZS (mislabel)

Head tube bore 44.0 mm; cups press fully inside the tube with only a thin lip showing; upper ZS44/28.6, lower ZS44/30 for straight 1-1/8 in steerers

Zero stack (ZS) headsets hide the bearing inside an oversized 44 mm head tube using a pressed-in cup, leaving only a low-profile lip visible. ZS44/28.6 is the single most common upper assembly on mountain bikes made since roughly 2010. On older straight-steerer frames the lower is ZS44/30.

Why it exists: Mountain bike makers wanted the low stack of integrated headsets without machining bearing seats directly into the frame; a pressed cup in a larger tube keeps the frame cheap to make and tolerant of wear.

Typically found on: Most mountain bikes 2010+, many gravel and hybrid frames, alloy e-bikes

Also called: Tapered zero stack lower, ZS55 (55 mm variant)

Lower head tube bore 56.0 mm; ZS56/40 accepts the 1.5 in base of a tapered steerer with a 39.8 mm crown race; ZS56/28.6 upper variants exist on some frames

The dominant lower assembly on modern mountain bikes: a 56 mm bore holds a zero-stack cup whose bearing clears a tapered steerer's 1.5 in base. Paired with a ZS44/28.6 upper it forms the most common MTB combination, ZS44/28.6 | ZS56/40. A slightly smaller 55 mm version (ZS55/40) appears on some brands' frames.

Why it exists: Tapered steerers needed a bigger lower bearing; ZS56 provides it while keeping the bearing tucked inside the frame for a short head tube.

Typically found on: Modern trail, enduro, and XC mountain bikes; many e-MTBs

Also called: 44 mm external lower, Tapered adapter lower

Head tube bore 44.0 mm; cup sits outside/below the tube; EC44/40 houses a 1.5 in tapered-steerer bearing under a straight 44 mm head tube; EC44/33 variants exist

An external cup pressed into the same 44 mm bore as ZS44, but with the bearing hanging below the head tube. EC44/40 is the trick that lets a straight 44 mm head tube (common on steel and titanium frames) accept a tapered fork: the oversized lower bearing simply lives outside the tube.

Why it exists: Frame builders using straight 44 mm tubing wanted one head tube that could run either straight or tapered forks by swapping cups.

Typically found on: Steel and titanium hardtails, custom frames, touring and bikepacking bikes

Also called: IS standard, Cane Creek IS, Standard integrated

Head tube bore 41.0-41.1 mm with molded 45-degree seats; drop-in cartridge about 41.0 x 30.15 x 6.5 mm, usually 45/45 (36/45 versions exist); IS41/28.6 upper, IS41/30 lower

An integrated (IS) headset has no pressed cup at all: the sealed cartridge bearing drops directly into an angled seat machined or molded into the frame, and you replace only the cartridge. IS41 is the Cane Creek-licensed version with a 41.0 mm bearing.

Why it exists: Integration shortens the head tube, cuts weight and stack, and gives carbon road frames clean lines; Cane Creek's version standardized one of the two competing bearing sizes.

Typically found on: Road and some XC frames from the 2000s onward; various carbon frames

Also called: Campy style, Hiddenset, Campagnolo standard integrated

Head tube bore 41.8 mm (SHIS rounds to 42); cartridge 41.8 x 30.15 x 8 mm, 45/45 contact; IS42/28.6 upper, IS42/30 lower

The integrated standard descended from Campagnolo's Hiddenset, with a 41.8 mm bearing — 0.8 mm larger than IS41 and not interchangeable with it despite looking identical installed. IS42/28.6 is by far the most common upper assembly on modern road and gravel bikes.

Why it exists: Campagnolo developed its own integrated geometry in the early 2000s; frame makers adopted it widely, and SHIS later named it IS42 to end the naming confusion with IS41.

Typically found on: Most carbon road and gravel bikes 2010+, endurance and race frames, many drop-bar e-bikes

Also called: 1-1/4 integrated lower

Lower head tube bore 47 mm; typically IS47/33 for tapered steerers with a 1-1/4 in (33 mm crown race) base; IS47/40 versions exist for 1.5 in bases

A mid-size integrated lower used on frames built for 1-1/8 to 1-1/4 in tapered steerers, a taper favored by some road and gravel brands as a lighter alternative to a full 1.5 in base. Bearing cartridges are about 47 mm OD.

Why it exists: Some designers judged a 1-1/4 in lower steerer stiff enough for road use with less frontal area and weight than 1.5 in, needing a matching in-between bearing seat.

Typically found on: Certain road and gravel frames (brand-specific), some aero road bikes

Also called: Integrated tapered lower, 56 mm-look integrated

Lower head tube bore 52 mm; IS52/40 cartridge about 52 x 40 x 7 mm, 45/45, over a 39.8 mm crown race; IS52/28.6 versions serve as oversized uppers for cable-routing headsets

The integrated lower that pairs with IS42 or IS41 uppers on tapered-steerer road, gravel, and mountain frames — IS42/28.6 | IS52/40 is the standard modern drop-bar combination. The same 52 mm seat also appears up top (IS52/28.6) on bikes that pass hoses through an oversized upper bearing.

Why it exists: Tapered steerers required an integrated-format bearing big enough for a 1.5 in base; 52 mm became the consensus size.

Typically found on: Carbon road, gravel, and XC frames with tapered steerers; most 2020+ drop-bar bikes

Also called: Tapered, 1.5 taper

Steerer is 28.6 mm (1-1/8 in) at the stem and 38.1 mm (1.5 in) at the crown, with a 39.8 mm crown race seat; 1-1/8 to 1-1/4 in tapers (33 mm crown race) also exist

Not a headset itself, but the fork format that shapes most modern headset choices. A tapered steerer needs a small upper bearing (any /28.6 upper) and a large lower one (/40, or /33 for the 1-1/4 taper). Typical full codes: ZS44/28.6 | ZS56/40 on mountain bikes, IS42/28.6 | IS52/40 on road and gravel, ZS44/28.6 | EC44/40 on straight-44 mm steel frames.

Why it exists: Puts steering stiffness where loads are highest (the crown) while keeping a light, standard 1-1/8 in stem interface up top — the compromise that ended the 1.5 in straight-steerer experiment.

Typically found on: Nearly all mid-to-high-end mountain, road, gravel, and e-bikes since about 2012

Also called: ACR (Acros/FSA), DCR (Deda), ICR, Headset routing

Usually an oversized upper bearing (often 52 mm, e.g. IS52/28.6) with a large inner diameter or ported compression ring so brake hoses pass between steerer and bearing

A late-2010s trend on road, gravel, and increasingly mountain and e-bikes: brake hoses and wires run from the bars through the headset's upper bearing into the frame. These systems need routing-specific headsets, compression rings, and split or profiled spacers, and are often tied to a matching stem and bar.

Why it exists: Aerodynamics and clean looks; hiding cables entirely required a path through the steering axis, which only an enlarged or ported upper bearing could provide.

Typically found on: Aero and endurance road bikes 2020+, performance gravel bikes, many current e-bikes; systems from FSA/Acros (ACR), Deda (DCR), Token, and brand-proprietary designs

By frame make, model, and year

The fastest route: look up your bike's spec sheet from the manufacturer, or enter the frame in Cane Creek's Headset Fit Finder, which lists SHIS codes for thousands of frames. Most brands publish the headset spec (e.g. 'IS42/28.6 | IS52/40') on the model's geometry or spec page.

By looking at the cups

Look at where the bearings sit. Visible cups stacked above and below the head tube like little hats = EC (external cup). Only a thin lip flush with the tube ends = ZS (zero stack). No metal cup at all — the bearing drops straight into the frame = IS (integrated). A knurled locknut with wrench flats and a stem that plunges into the fork = 1 in threaded. Check top and bottom separately; they are often different types.

By measuring

With the fork out, use calipers to measure the head tube's inside diameter at the top and bottom — round to the nearest whole mm (44, 42/41.8, 41, 34, 47, 49, 52, 56). Measure the steerer at the stem clamp (25.4, 28.6, 31.8, or 38.1 mm) and the bare crown race seat at its base (26.4, 27.0, 30.0, 33.0, or 39.8 mm — 39.8 is written as 40 in SHIS). For IS frames, measure the old cartridge bearing: OD x ID x height, e.g. 41.8 x 30.15 x 8. Distinguishing IS41 (41.0 mm) from IS42 (41.8 mm) requires this care — they look identical installed.

By markings

Many headsets have the SHIS code laser-etched on the cup or top cover (e.g. 'ZS44/28.6'). Sealed cartridge bearings usually print their size and contact angles on the rubber seal, such as '41.8 x 30.15 x 8 45°/45°' or a code like MR122/MR171 — photograph the seal before buying a replacement. Cane Creek, Chris King, FSA, and Wolf Tooth all etch model info you can cross-reference on their sites.