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I ended up in a situation in which my new fork with an integrated crown race imposes lower headset bearing (1.5" 45deg) stack restriction, i.e. I need an additional 1.5mm of bearing height so the fork doesn't rub lower headset cup (bonded to CF tube, by the way).

I see two ways to alleviate the issue:

  1. Grind a bit of headset lower cup so the fork clears. It's aluminium and hanging part that rubs the fork serves no structural purpose, at least that's what I think.
  2. Use a shim between lower cup and a bearing. See the CAD drawing for details. I reckon the support of cup from top (45deg surface) and outside (54mm inner diameter) should be enough, after all it's the same mechanism that keeps the bearing in place. But to my surprise, I failed to find such a product!

shim CAD drawing, not actual size

So, the question(s): is the shim approach a viable solution? If no, why exactly? If yes, can I buy one?

Update: in perfect scenario, I want to use the shim with another, less tall but lighter bearing I already own. I do not want to purchase a taller bearing.

Here's a fork crown photo: fork crown A the tight situation: enter image description here

Update 2: as promised, the bottom headset cup: bottom headset cup The tall bearing height is 8mm, the less tall lightweight bearing is 7mm.

Update 3: the 2.5mm shim looks too thick even for a 7mm bearing, the gap is ~2.4mm: 2.5mm shim 2.4mm gap with 2.5mm shim and 7mm bearing

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    A picture of the lower headset cup and fork crown may be helpful here. – Argenti Apparatus Mar 11 '18 at 14:28
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    @ArgentiApparatus added the fork photo. Maybe I'll add the cup tomorrow too (I'd have to press the bearing out and that's a bother.), but you might as well just imagine a regular CF front tube with an integrated 1.5" 45deg cup. – Klaster_1 Mar 11 '18 at 14:36
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    You may need a thicker bearing. Cane Creek has a bunch of helpful charts to help with proper bearing selection. – BillSkiCO Mar 11 '18 at 20:04
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    What fork is this? This is a really weird situation and I believe it's the fork that's the root problem. Usually they have more tapered area than this. Putting a shim in the frame will make it challenging to keep the bearing seat as precise as it's supposed to be. It may lead to some combination of problems with noises and not being able to adjust the headset well. – Nathan Knutson Mar 12 '18 at 5:25
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    @NathanKnutson let's say this fork comes from a completely different frameset. Yes, I am mix-matching components. – Klaster_1 Mar 12 '18 at 6:08
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The custom shim works, I settled with a ~1.4mm option, this was the minimal thickness machinist agreed to make, he said "it would bend in your pocket otherwise". After properly preloading the headset bearings I did not notice any play during pedaling efforts or when applying front disc brake during ~200km the bike made since assembly. That's not much distance, so if anything happens and I survive I'll update the answer.

Apparently, some people experience the same problem. To get your own shim, find a local machinist and use the blueprint from the question, adjust thickness as needed. My shims were made from a 2024 aluminium alloy plate. The outer diameter tolerance should be in the ballpark of +0mm -0.03mm, the rest doesn't matter.

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I bought a bike from Van Nicholas in the Netherlands. It's the Skeiron road bike. The frame came with the fork so I would guess it should have been machined together for bearings and all without a problem. They sent 2 shims same as you described but I can't see why 2. The fork fits nicely, very close, but nicely with the Cane Creek headset bearing in the crown. The top of the headset was a different matter however, the headset cover did rub the headtube and one of those shims placed between the bearing and the headtube race made for a prefect fit. Almost a year of riding and no issues at all.

  • That's interesting, can you add a shim photo and maybe shop contact info? – Klaster_1 Nov 5 '18 at 2:57

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