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I replaced my stem with one that has a taller stack height. I learned about proper gaps for the headset preload and had to requisition a thinner spacer to ensure there's ~3mm between the top of the steerer and the stem spacers. (I initially had the top cap too far from the top of the stem and did a 10 mile trail ride like that)

After installing the thinner spacer and torquing everything down, I noticed the dust cover/top cap of the headset bearings has a gap. It's flush on the front of the bike and ~3mm uncovered on the back. I disassembled it, cleaned out all the muck, and put it back together (tightening the top bolt first, then adjusting the stem position, then final torquing the stem bolts), but can't figure out why the bearing and split-ring washer thing don't go into the head tube far enough for the dust cover to be flush with the frame. The teen at the bike shop said sometimes you have to tap them in with a hammer. Is this correct?

There's no binding I can feel, but the front occasionally feels like it wanders on my test drives--like the stem/steerer connection is too loose. I can't tell if this is all in my head, or if the bearing slightly protruding from the headtube is indeed the culprit.

When I got the bike this spring, the headset/head tube made a ton of creaking and groaning, which was solved by disassembling and greasing everything with Park's PPL-1 lube. I've learned a lot more about bikes maintenance since then, but don't remember having any leftover parts after reassembly!

What have I done wrong? I can't find any 2020 Specialized Rockhopper parts diagrams to confirm whether a spacer or something is missing. The bearings pretty obviously only fit in one way, same for the split ring guys on the top and bottom. But maybe I have a compression ring upside down?

Stem and spacers Gap in headset dust cover

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    I believe general wisdom is to always have at least one spacer below the stem. That the underside of your stem is not flat makes me wonder if there's a special spacer supplied with the stem that you don't have ? – Criggie Dec 30 '20 at 23:29
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    @Criggie that was my thought. That marking on the stem probably means it's a PNW components Range stem. However, the site doesn't have a technical FAQ or manual for the stem, and the pics don't let me see if the underside is supposed to be curved like that. It is an odd shape, and I don't know this shape to be a standard practice on MTBs. pnwcomponents.com/collections/pnw-components-stems/products/… – Weiwen Ng Dec 30 '20 at 23:33
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    On the top, the spacer sits in a depression that allows it to sit flat. I suspect the bottom looks the same? That is, a spacer will sit flush in a way that the dust cap does not. It may not even move the stem up very much from where it is now because of the inset – Andrew Dec 30 '20 at 23:43
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    Bontrager mandates at least 5mm of spacers under the stem for structural/safety reasons. The spacers allow a larger portion of the steerer tube to flex when handling loads, instead of all the force being sharply concentrated between the stem and headset bearing. I would send PNW these photos and ask what they have to say. Their customer service is excellent. – MaplePanda Dec 31 '20 at 1:02
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    It looks to me like the one spacer on top is only contacting the stem at the front, due to the top surface of the stem also being curved. Seems like the preload was only applied at the front of the steerer tube because of this and the stem's bottom curve. – Armand Dec 31 '20 at 23:40
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I think the answer is that the Range stem needs spacers with an outside diameter small enough to fit inside the "shelf" ridge inset into the top and bottom of the stem (the depression that Andrew noted in his comment). This shelf is perpendicular to the steerer axis, not sloped, so any spacers properly seated on it will be flat and flush. This also means that at least one spacer is required beneath the stem, as it looks like the headset top on the Rockhopper will not seat properly in that shelf.

Here's a view of the PNW Range stem I found online that clearly shows the shelf: enter image description here

Just for reference, here's a promo photo of the stock Rockhopper 2020 from Specialized: enter image description here

If you click to open the image and zoom in, you can see that the headset top is flush with the headtube top, and the spacers above that along with the stem all seem to be flat and flush. This is consistent with Criggie's observation that the OP's bike photos show a stem with curved bottom and top surfaces, which suggests the root of the problem.

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    Excellent find, makes a great answer, well done! – Criggie Jan 1 at 3:09
  • Thanks for the detailed answer, Armand. I'm waiting to hear back from PNW about requiring at least one spacer below the stem, but I tried moving my thinner spacer down there, with no change in the gap. I disassembled it again and checked that the compression ring is installed correctly, pressing it as far down as I could, but the gap persists. The cap also turns with the steerer, which seems wrong. Shouldn't it remain in place against the head tube, with the steerer rotating inside it? – Eric L. Jan 4 at 18:53
  • @Eric your spacer outer diameter is too big, so it isn't fitting inside the "shelf" on the stem. You'll need to buy spacer(s) with a small enough outer diameter to fit inside that shelf. Easiest thing to do is to take the stem to your local bike shop and make sure any spacers you buy will fit. I think you'll need to buy at least one new spacer for each of the top and bottom of the stem. (I'm saying the spacers you have are too big - outside diameter - so don't use them with this stem -- put them away in a drawer) – Armand Jan 4 at 22:12
  • @Eric Regarding parts moving: the headtube top is obviously fixed, while the stem obviously turns with the steerer; somewhere between the two is the spot where moving part is adjacent to fixed. That happens inside the headset to have bearings and lube between stationary and moving parts. Thus, the top of the headset will move with the steerer/stem. Depending on the headtube design, part or most of the headset may be hidden down inside the headtube. I think that's the case with your bike, so only the very top rotating part of the headset (what you call the cap) is visible above the headtube. – Armand Jan 4 at 22:38

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