As above really. I am putting on a longer travel fork with a shorter steerer. The bike has a tall external cup. Can I fit a zero stack to get more steerer clearance to fit the stem? Geometry should be closer to standard if I could too.

I.e. swap a EC44/28.6 for a ZS44/28.6

I don’t see why not. You are only replacing the bearing cup holder for a different style.
Don’t know about the steering tube fit but why would it be different?
If they both require a press/interference fit then should be the same.
See the Hope Headset data sheet here as a reference

https://www.hopetech.com/_repository/1/documents/HOPEHEADSETS2011Web.pdf

On a 44mm head tube, normally, ec is for a 1.5 tapered fork, zs for 1.125.

You can fit the 1.125 in an external cup but it usually needs a reducer.

If you’re planning on running a tapered fork (the two you mention are 1.125) then no, no zs cup as there’s insufficient room.

You can claw back about 7mm on the top using a staggeringly expensive cane creek slam set.

In a 44mm head tube you can with an old skool straight steerer (which it sounds like you have), you can’t with a tapered fork

In addition to those wise words… if your frame is designed to use a lower external cup, then moving to an internal lower cup could well result in crown mounted adjusters hitting the down tube. The top cup is fair game.

If you do get crown-adjuster / down-tube interference, you could potentially fit an upper headset with an integral block mechanism, Acros produces one as, I think, does Ritchey.

But yes, as above, it should work with a straight steerer 1 1/8 fork, but not with a modern tapered steerer as the head-tube simply isn’t wide enough for the bearing assembly.

Do they have a low stack height though BWD? Or will they just add back in the height saved by changing the lower cup?

Oh, @22jeffers , what stem are you using? Could that not be swapped to one with a lower stack height?

Daft question, why are you wanting to fit a longer travel fork then contemplating doing something which will increase the head angle to offset the change? (assuming a 12mm stack on your external cup a true zero stack would almost completely offset the length change from a 20mm longer same model fork at 30% sag, switching from say a pike to a fox 36 you’d actually end up 2mm lower on paper)

Do they have a low stack height though BWD? Or will they just add back in the height saved by changing the lower cup?

They’re not too bad tbh, at least the Acros one isn’t. But yes, I guess if the primary motive is to allow the use of a shorter steerer, it might not work. Then again, there’s a reasonable chance the adjusters won’t hit the downtube anyway, particularly if it’s curved for additional clearance.

Daft question, why are you wanting to fit a longer travel fork then contemplating doing something which will increase the head angle to offset the change?

I think it’s because it’s as much about being able to use a shorter steerer as it is anything else.

BWD Yep that would be my suspicion too but a shorter bottom cup seems the last place you’d try find that length though after a shorter top cup, smaller stack stem, less spacers etc.

Assuming the “new” fork doesn’t have a 30mm shorter steerer or something daft it’s likely to be reclaimable up top easy enough.

Top or bottom half of the headset? Noted that everyone seems to be assuming that you want to change the lower cup, but noted that the OP finishes with “I.e. swap a EC44/28.6 for a ZS44/28.6” – the “28.6” bit is the metric version of 1 1/8″, and suggests top half of a headset. Even with a straight 1 1/8″ steerer, there’s a (slight) flare at the base of the steerer – otherwise the crown race would just flop about on the steerer.

Top or bottom half of the headset? Noted that everyone seems to be assuming that you want to change the lower cup, but noted that the OP finishes with “I.e. swap a EC44/28.6 for a ZS44/28.6”

I thought EC44 was only on the bottom. Is there such a thing as a top EC44?

Is there such a thing as a top EC44?

Yes. See, for example, slacksets.

Even with a straight 1 1/8″ steerer, there’s a (slight) flare at the base of the steerer

You’re right of course and it should be a 44/30 but, for my part, I’ve never seen an ec44/28.6 cup that isn’t actually a /30 marked for ease. Happy to be contradicted but I don’t think there is any such thing as an external top cup for 44mm (unless you’ve a 1.5 steerer or possibly overdrive?)

Geometry should be closer to standard if I could too.

A top cup wouldn’t do that.

Edit: beaten to it, and answered. Yes a slack set would be the right answer and explain the geometry change from a top cup. Though given the op’s question I doubt a slack set is the right answer simply because the chances of having a slack set knowing what it is and how it works but not knowing it replaced a zs44 are I suspect, slim.

Perhaps OP currently has a slack set with EC44/28.6 top headset and a short fork, plans to achieve same geometry by switching to a non-slack set with ZS44/28.6 top headset and a longer *edit* travel */edit* fork (albeit with shorter steerer)?

Yep, straight 1 1/8” steerer and yes, I’m just talking about the top cup. It has a really tall cone covering the bearings at the top.

I have a 2018 giant ATX. The factory headset part number is 1310-NO11Q8-101. All I can find out about the factory headset is that it’s listed as 28.7-44-30 height 10.7

So I’ve assumed this is actually an EC44/28.6 upper cup.

Is it not that weird Giant overdrive.

If its 44 ht 28.6 , 1 1/8 steerer zero stack will work.

has a really tall cone covering the bearings at the top.

You can probably just ditch that for a short cover.
I’ll lay good odds it’s a zs with a high top like this one https://www.evanscycles.com/pro-ud-carbon-20mm-is-top-cover-EV213653

Post a photo of your current headset if you’re not sure but one of these

https://activesport.co/Hope-Headset-07-Top-Cover-2007-Onwards

sorts of things will probably be what you’re looking for, much easier and a bit cheaper than a new top cup.

Your part number above returns the following fsa unit on Google. It’s a zs44/30 bottom and a zs44/28.6 top so unless it’s been changed its already zero stack.
https://www.h2gear.co.uk/17455/products/giant-no11-agytc-1-18bk286-414-30h91-headset.aspx

Thanks, that’s really useful. Yes it does look like that cap. I’ll look for a picture. Cheers.

I assume I can’t post pictures direct on here as I can’t see how to do it. I’m struggling to find a link to a good picture but you can just about see it if you zoom into this one:
ATX

The bike in the image has ZS cups top and bottom, no EC in sight. You need to change the top cover, as mentioned above.

Ah! Great news.

Thanks everyone and apologies for the wild goose chase!

You’re welcome, we’re fine with goose chases here, so long as it’s a free range organic wild goose that’s led a happy life.

Worth checking that your bars / shifters don’t hit the top tube on full lock or you risk dinging it in a crash. You may need to adjust the angle of the shifters or consider a higher rise bar or stem. Or just don’t crash 🙂

2018 Giant? Is the original fork (and headset) Overdrive 2 with a 1 1/2″ to 1 1/4″ tapered steerer – if so, this would explain why you’d need an external cup in a 44mm headtube. If new fork uses a standard 1 1/2″ to 1 1/8″ tapered steerer, then should be able to go ZS in a 44mm headtube.