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Has anyone had an expander bolt that when fully tightened (I.e. bottomed out) didn't expand enough to grab /hold the tube?

I'm installing a Shimano/PRO Di2 battery holder / expander bolt and I can't reach the 7 nm tightening torque. As I tighten the bolt, the expander ring starts to expand and then stops when it reaches the bottom. Any more torque causes the expander to spin in the tube.

The expander is designed for 1 1/8 steer tube, which my bike is. Is it possible that the wall thickness on my steer tube is a fraction of a millimeter thinner than other bikes.? It's a steel tube so is it possible that this expander bolt was designed for carbon steer tubes that have thicker wall thicknesses?

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You're thinking the right things. Even a basic steel threadless steerer is much thinner-walled than any aluminum one, and carbon steerers can have very thick wall thickness. This battery holder is basically a carbon plug with some other stuff added. Carbon plugs that fit on the gamut of commonly encountered carbon steerers aren't typically going to work on a steel fork.

You want to try to bulk up the silver knurled aluminum expander bits or the inside of the steerer, but you need to do in a way where once it's all tightened, it still holds when you go to adjust your headset preload.
Foil tape is around 0.08mm, so 0.16mm per layer in this application. If around .5mm sounds like it might be enough, you could put three layers of it inside the steerer or on the faces of the knurled bits and see if it bites then.

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Certainly plausible. Your steerer tube could be extra-thin and the weight saved by shaving out the inside, or it could have been eroded in the middle over time.

There are specialty tools for measuring inside bores, but you're unlikely to have one or access to them. It may work to use a circle of cardboard as a gauge, or compare two tube's internal diameters with a suitable finger.

Solutions and workarounds:

Can you flip the expander? Mount it higher or lower in the steerer?

If this expander only holds the battery, then it may be feasible to shim it using brass or another compatible material.
I would NOT shim if this pliug is doing anything for your steering.

It is conceivable that your plug is also undersized - a set of calipers may help prove that. Everyone technical should own some calipers.

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    can't you measure the bore here with the other jaws of the calipers? Or is it not a parallel bore all the way down? (haven't taken bars off a bike for ages and have no experience with high-end stuff so I could easily be talking nonsense)
    – 2e0byo
    Aug 4, 2022 at 9:36
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    @2e0byo yes calipers would be ideal for measuring the internal diameter, but that's at the mouth/end. Its possible the tube is not consistent throughout its length and measuring the width down in the middle would be pretty hard. A telescoping bore gauge of the right size might work but again hard to get out if the end is smaller than the middle. Like measuring the width of a barrel in the middle vs the end.
    – Criggie ♦
    Aug 4, 2022 at 10:58
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    ah right. Yeah, unless the OP has a dial bore gauge (and the difference is within the travel of the dial) or he can somehow wiggle a standard bore gauge out without knocking it, then it's improvisation or guesswork. (I wonder if you could show it with a laser pointer set up to just intersect the tube at each end?) Anyhow, missed that the shaving out is likely in the middle.
    – 2e0byo
    Aug 4, 2022 at 15:41
  • @2e0byo I think Nathan's answer is better - the wall thickness will also depend on the design, with steel plausibly being thinner than the CF that the battery holder was designed for originally.
    – Criggie ♦
    Aug 4, 2022 at 21:38

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