Aluminium alloy bottle cage bolts are universally bad and should be banned. I cannot be moved from this position that I feel unreasonably passionate about.
It is alarmingly common to find alloy bolts – typically with a smooth, mushroom/button-style 3mm socket head – supplied on high-end bikes.
As far as I can ascertain, bike brands use alloy bolts to reduce weight and to mitigate the minor increased chance of galvanic corrosion when using steel bolts in aluminium alloy inserts.
The difference in weight between an aluminium alloy and stainless steel bolt is so small that it's entirely negligible in the real world – that paltry ~2g difference matters for absolutely nothing in the context of a whole bike.
And as I'll come on to, the galvanic corrosion point is moot with correct preparation – preparation that is essential for an aluminium bolt, too.
The only (barely) justifiable rationales for using aluminium alloy bolts put to bed, let’s cover the litany of significant practical problems they introduce, and how we got here in the first place.
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Stuck with no way out
These are not just the maddened ramblings of an irked cycling journalist – Ben Meir, general manager of Reap Bikes, agrees: "Brands are prioritising the explicitly measurable – weight – over real-world practicality, long-term durability, and sustainability."
When two dissimilar metals meet, galvanic corrosion can occur. This means a bolt can effectively weld itself into the typically metal eyelets pressed or welded into your frame. This is, in part, the same phenomenon that causes stuck seatposts.
Stainless steel and aluminium alloy are dissimilar metals, so mixing the two, in theory, increases the risk of galvanic corrosion occurring.
This is why it’s crucial that all bottle cage bolts – aluminium alloy or stainless steel – are greased before fitting.
The difference is that, should the bolt seize – even a little bit – the stainless steel bolt is much less likely to shear or round during removal.
The aluminium alloy typically used for bottle cage bolts is softer than stainless steel. Speaking from tiresome experience, it’s very easy to round out the head of the bolt, even with a sharp hex wrench.
Worse still, the head can even snap clean off (this has happened to me), leaving you with a stubborn stump firmly lodged in your frame's bottle boss.
“I understand the reasoning for using an aluminium alloy bolt with an aluminium alloy insert,” says Meir. “When steel and aluminium alloy are together, the steel comes off better – and if the aluminium alloy is in the frame, that’s an expensive issue to resolve."
Meir is critical of the industry’s obsession with weight savings that led to this situation in the first place.
“The relentless chasing of weight – so brands can claim x per cent lighter than last year – has likely led to aluminium alloy inserts, which leads to aluminium alloy bolts, which leads to hair-pulling by riders,” he says.
Reap opts for brass inserts specifically to avoid this problem, even if they do come with a minor weight penalty: "If any galvanic corrosion occurs, it’s the bolt that needs replacing, not the frame.”
Mushroom madness
Material aside, if a bolt does seize, it is nigh-on impossible to get a grip on the pesky mushroom/button heads bafflingly favoured by bike OEMs.
Sitting flush against the baseplate of your bottle cage, they offer next to no surface area for tools such as mole grips to grab onto.
This is true of stainless and aluminium alloy bottles, but it’s doubly bad if they’re made from aluminium alloy – their softness means they'll deform rather than budge.
It’s sometimes possible to cut a slot across the head (if you're careful, and have a Dremel to hand) and use a flat-head screwdriver to remove them, but this is a high-risk strategy.
There is a better way
No matter what the insert is made from, a far simpler solution exists – just use a 4mm socket, standard stainless steel cap head bolt, and grease the damn thing.
While the risk of galvanic corrosion is higher when using a stainless bolt in an alloy insert, I have never had problems fitting or removing bottle cages, provided they've been prepared with a healthy dollop of grease.
You’re also far less likely to round out a seized bolt in the first place – cap heads have a much deeper and more secure head for your hex key to fit into than a mushroom head.
Should worst come to worst and you round the bolt, you have more options for removal with a cap head.
At the most basic, you may be able to grasp the head with a set of slender jaw mole grips or pliers, and have a crack with that – something that's not possible with a mushroom head bolt.
If push comes to shove, drilling the head off is much easier with a cap screw than a mushroom head.
The only possible downsides of stainless cap head bolts?
As Meir explains, if a stainless bolt is really stuck in an alloy insert, you may risk damaging the insert with vicious removal – a costly mistake. Repair is possible, but best avoided.
I can't say I've found this necessary, but the paranoid who enjoy riding in dank weather may consider regreasing bolts yearly.
Secondly, almost all bottle cages have recessed slots that the bolts sit in, fully enshrouding a regular cap head bolt without snagging on your bottles. If your cage doesn’t work with cap head bolts, get different cages.
Now I’m paranoid about my bottle cage bolts. What should I do?
SQUIRREL_13214774
If your current or future bike has alloy button head bolts, do future you a favour and blow £5 on a large pack of high-quality stainless steel M5 cap head bolts.
Install them with a good dollop of grease, and then forget about them completely. You’ll save yourself a huge headache down the line.
If you’re worried about weight, I invite you to perform a back-to-back test on your steepest hill with your alloy bolts, followed by stainless steel, and share the results below.
I can’t believe I’ve just written 1,000 words on this subject, but some hills are worth dying on.
