ZS44/EC44 Headset: Can Your Frame Take a Tapered Fork?

A frame with a 44mm head tube can take a tapered fork without any machining — but only if the lower bore actually measures 44mm inside diameter. When it does, a ZS44/EC44 headset does the conversion: the zero-stack upper cup stays inside the head tube for the straight 1-1/8" (28.6mm) steerer, and the external lower cup adds the larger 39.8–40mm bearing bore a tapered 1-1/8"–1.5" steerer needs, without the head tube itself having to be any bigger than it already is. If the bore measures something other than 44mm, ZS44/EC44 is the wrong part number, and buying it wastes the trip.

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What ZS44/EC44 actually means

Headset codes follow the Standardized Headset Identification System (SHIS), introduced by Cane Creek in 2010 and used industry-wide since. The format is [cup type][head tube bore]/[steerer diameter], with the upper and lower specs separated by a slash or vertical bar.

• ZS = Zero Stack. The cup presses fully inside the head tube; nothing sits proud of the frame.
• EC = External Cup. The cup’s neck registers into the head tube’s bore, but the body of the cup — and the bearing it holds — sits outside the tube, external to it.
• 44 is the head tube’s inside diameter in millimeters, rounded to a whole number.
• The number after the slash is the steerer/crown-race diameter: 28.6mm for a standard 1-1/8" upper, and 39.8–40mm for the 1.5" lower a tapered fork uses.

So a ZS44/28.6 | EC44/40 headset is read as: zero-stack cup in a 44mm bore up top taking a 28.6mm steerer, external cup in the same 44mm bore at the bottom taking a 40mm crown race. Both cups register into the same 44mm hole — the head tube itself doesn’t change shape — but the external lower cup physically extends outward to make room for the bigger bearing a tapered steerer’s lower section needs. That’s the entire trick behind this conversion, and it’s why it only works on a frame whose head tube is a straight, parallel 44mm bore from top to bottom, not one that’s already flared.

Measure the bore before you order anything

Frame marketing copy that says “44mm head tube” is not proof the lower bore is 44mm — some frames use a 44mm top and a larger bore at the bottom by design, in which case they were built to take a tapered fork directly and don’t need a conversion cup at all (more on that below).

Cane Creek’s own headset identification guide lays out the procedure: remove the existing headset cups and bearings, then measure the inside diameter of the head tube at both the top and bottom openings with digital calipers, rounding each reading to the nearest whole millimeter. Do this at both ends even if the frame is advertised as a single size — the two ends are not always the same.

Common readings and what they mean:

• ~41.8mm — this is the older ZS41/IS41 standard used on a lot of 2000s-era road and hybrid frames. ZS44/EC44 will not fit; the bore is too small for the cup’s register.
• ~44mm at both ends — a parallel 44mm tube. ZS44/EC44 is the right conversion.
• ~49mm at the bottom — a parallel 49mm tube, one size up. This needs an EC49 lower cup, not EC44.
• 44mm top, 52mm or 56mm bottom — the tube is already physically tapered. It was designed for a tapered fork from the factory, and it needs a standard tapered headset (ZS44/ZS56, for instance), not a conversion.

If the bore reads 49mm instead of 44mm

Cane Creek also sells a 40-Series ZS44/EC49 Tapered Conversion Headset, built the same way as the EC44 version — zero-stack top, external cup bottom — but sized for a 49mm parallel bore instead of 44mm. It’s easy to grab the wrong one by memory alone, because both are marketed as “tapered conversion headsets” and both take the same 1-1/8"-to-1.5" fork. The difference is entirely in the lower cup’s outside register, which has to match what calipers actually read on the frame, not what seems like the more common size.

If the head tube is already tapered in shape

If the top and bottom bores measure two different sizes on a straight-through reading — most commonly 44mm and 56mm — the frame doesn’t need a conversion headset at all. It was manufactured with a genuinely tapered head tube, and both cups are zero-stack (ZS44/ZS56), seated fully inside the frame with no external cup anywhere. Buying an EC44 conversion set for this frame is the wrong move twice over: the lower cup’s 44mm register has nothing 56mm to press against, and there’s no external-cup step to bolt onto in the first place.

What the headset conversion doesn’t cover

Confirming the bore settles whether the frame can physically take a tapered steerer. It doesn’t settle whether the fork itself will work on the bike. Check separately, before ordering the fork:

• Crown race diameter. Most 1.5" tapered forks use a 39.8–40mm crown race, matching the EC44/40 or EC49/40 lower cup. A handful of carbon forks mold the crown race into the steerer itself and aren’t compatible with a separate press-on crown race at all — White Industries flags this explicitly on its own ZS44/EC44 spec sheet, and it’s worth checking your specific fork’s documentation before buying.
• Axle standard, brake mount, and travel. None of these are headset questions. A tapered steerer says nothing about whether the new fork’s thru-axle spacing, disc mount, or axle-to-crown length suit the rest of the frame — those are separate compatibility checks entirely, the same ones that come up when fitting a 140mm 29er fork to a 27.5 frame.
• Stack height. ZS44/EC44 conversion cups typically add a small amount of stack (commonly around 8mm up top, 12mm at the bottom) compared with a fully internal ZS44/ZS44 straight setup. On a frame with limited steerer length to spare, that’s worth accounting for before cutting anything.

If the head tube measures anything other than a clean 44mm parallel bore, don’t force a ZS44/EC44 kit onto it — the cup either won’t seat or won’t hold, and a headset that isn’t fully seated is a safety issue, not just a fit issue. Match the code to what the calipers actually say, not to what the frame was marketed as.