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I have a couple of old aluminum frames and the bottle cage mounts always show signs of galvanic corrosion due to the steel rivnuts. Why do manufacturers not use aluminum rivnuts?

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    Assuming people use steel bolts, wouldn’t you get the same issue but between bolt and rivnut?
    – Michael
    Mar 4, 2022 at 8:13
  • @Michael I would assume that the relatively less contact duration due to instances of removal and the grease re/application would somewhat inhibit the battery effect
    – Gregory Leo
    Mar 4, 2022 at 8:27
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    They are cheap and manufacturers sell aluminium frames with a 5-year lifespan in mind.
    – Noise
    Mar 4, 2022 at 8:41
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    *I would assume that the relatively less contact duration due to instances of removal and the grease re/application would somewhat inhibit the battery effect * For most riders, bottle cages are fit-and-forget. No regreasing (and anyway greasing threads doesn't provide a complete insulating barrier. No removal.
    – Chris H
    Mar 4, 2022 at 11:10
  • Does this apparent corrosion cause any problems? It's never made them loose on any of my bikes, for example
    – Chris H
    Mar 4, 2022 at 15:02

2 Answers 2

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Steel has a much broader plastic deformation range with a well defined yield point, while for aluminum there is practically no plastic zone and the difference between end of elastic zone and fracture is very little.

This means that an aluminum rivnut would always "bounce back" or reach a state very close to fracture, which is not what one desires for this use case.

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    if aluminum isn't a suitable material for rivnuts, then I'm confused as to why aluminum rivnuts exist
    – Gregory Leo
    Mar 4, 2022 at 12:44
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    @GregoryLeo, there are cases where you want the failure to happen, to protect the part where they are applied
    – L.Dutch
    Mar 4, 2022 at 12:59
  • @Gregory - also the likes of aviation where manufacture and installation is not as cost focused and done to precise tolerances.
    – mattnz
    Mar 5, 2022 at 1:50
  • @GregoryLeo Apparently, aluminium rivnuts are less likely to damage fibreglass when used in that.
    – Andrew Morton
    Mar 5, 2022 at 18:03
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I'd suspect the strength of the threads is the main reason.

Aluminium threads are known for stripping easily, and torque maximums must be observed. Steel threads are much more robust.

Personally, if I'm installing a rivnut, I slather the outside with mixed epoxy on the basis this will fill in all the voids and help isolate the dissimilar metals. You can also use assembly lube as an isolator.

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  • There are plenty of aluminium bolt holes on bicycles. Stems, mounting holes for racks/mudguards at the dropouts etc. So I’m not sure if the strength argument really holds.
    – Michael
    Mar 4, 2022 at 10:26
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    @Michael that might be a good question - why do we see rivnuts used for all water bottle cage mounts, but not so much for other threaded holes?
    – Criggie ♦
    Mar 4, 2022 at 10:48
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    @Michael I do suggest you ask that question, but here's a hint: how many of those other threads are into the walls of tubes?
    – Chris H
    Mar 4, 2022 at 11:08
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    @ChrisH: Yes, you’ll need some kind of rivnut or other reinforcement for threaded holes in (thin) tubes. But when it comes to thread strength it should be sufficient to make them out of aluminium (as demonstrated by stems etc.).
    – Michael
    Mar 4, 2022 at 14:34
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    @Michael they'd need to be have much fatter walls than the steel or even brass rivnuts I've used. There's plenty of meat to work with in stems, dropout plates etc.
    – Chris H
    Mar 4, 2022 at 14:59

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