25/09/2026
Have you found yourself in Top Mount bolt hell..!!
This is a very geeky explanation as to why you’ve snapped your Top Mount Bolt…
I am often delighted by the engineering found in the i3. Working on them is genuinely a pleasure (anyone worked on a modern JLR product!!).
However, sheared Top Mount bolts are the result of frankly, bad engineering. Steel bolts in aluminium don't play nicely together at all...
If you're thinking of DIY'ing your Top Mounts or if you’ve already broken one, read on:
Top Mount bolts are fitted hand tight (28Nm) this means if they are going to come out, they can be removed easily by hand using a small socket wrench.
If yours won't move at all or quickly become tighter, you're in trouble. I can’t stress this enough.. they should come out very easily. Don't continue to add torque or resort to a power tool. You will just snap them off.
Applying heat (don't set your car on fire) and gently tightening and loosening the bolt can free the corrosion. Be patient.
I haven't had much luck with penetrating fluids, even when left overnight, but this is worth trying to avoid a breakage.
If you do break one or more, there are individuals whose business is removing sheared fasteners. Google ‘thread repair’ service. I don’t recommend you try drilling these out yourself. Without a suitable drill guide and the correct tools, you’re likely to make things much much worse.
Looking at the picture I posted, you can see things get pretty nasty up there. There are actually three very detrimental processes that this engineering design failure promotes.
Galvanic Corrosion, Fretting Corrosion & Crevice Corrosion.
Here's a quick deep dive (courtesy of Gemini) as to what is happening inside your Top Mount. The question, why, when well known, basic electrochemistry is involved was this design approved??
Galvanic Corrosion
The corrosion that occurs between steel and aluminium is called galvanic corrosion. It is an electrochemical reaction that occurs when two different metals come into direct electrical contact in the presence of an electrolyte, such as moisture, rain, or saltwater. Saltwater, high humidity, and acidic or alkaline conditions drastically increase conductivity, speeding up the reaction.
So a wheel arch, sprayed with salt water, would count as the perfect environment to promote this phenomena.
In a steel-and-aluminium pairing, aluminium is more active (anodic) relative to structural steel (cathodic). As a result, the aluminium acts as a sacrificial material and corrodes at an accelerated rate, forming a white, powdery residue (aluminium oxide/hydroxide).
BMW did use a coated, plated fastener (these things are used to minimise the potential difference against the aluminium), but it would appear that the environment this fastener works in is highly active and the coating just isn’t enough.
Crevice Corrosion
While galvanic corrosion happens between two different metals, crevice corrosion occurs between two pieces of the same metal when liquid gets trapped in a narrow gap, joint, or overlap.
This is exactly what is happening between the upper surface of the top mount (aluminium) and the alloy chassis leg (also aluminium) that it is bolted to.
How Crevice Corrosion Works
Aluminium naturally forms a microscopic protective layer (aluminium oxide) when exposed to air. However, inside a tight gap between two bolted plates, a self-sustaining electrochemical reaction takes over:
Oxygen Depletion: Water or moisture seeps into the void. The oxygen inside the narrow gap is quickly consumed by initial surface oxidation and cannot be replenished easily because fresh air is blocked.
Differential Aeration Cell: The aluminium exposed to open air remains oxygen-rich and acts as the cathode. The oxygen-starved aluminium inside the gap becomes the anode.
Acidification & Breakdown: As aluminium ions (Al^(3+)) dissolve into the stagnant moisture inside the gap, they react with water to form acidic compounds (hydrochloric acid if chloride/salt is present). This acidic micro-environment destroys aluminium's protective oxide film, causing aggressive pitting and metal loss inside the joint while the outside looks completely normal.
Fretting Corrosion / Exfoliation
If the two aluminium plates experience micro-vibrations relative to each other (common in automotive or aerospace joints), the friction constantly rubs away the protective oxide layer, accelerating the moisture-driven decay.
Pack Rust / Oxide Jacking
As aluminium corrodes in the void, the resulting aluminium oxide powder occupies 3 to 5 times more volume than the original metal. This expansion creates massive internal pressure, pushing the two bolted plates apart and bending or shearing the bolts.
So, if you’ve made it this far.
The steel Top Mount bolts experience Galvanic Corrosion. Aluminium oxide builds up binding the bolt into the chassis leg.
In addition, and in more extreme cases. Crevice corrosion occurs filling the voids in the chassis leg with more aluminium oxide. In the most extreme cases, oxide jacking occurs. Large amounts of aluminium oxide build up between the top mount plate and chassis leg surface. As described, this creates a huge pressure that forces these components apart, jacking or tearing at the bolts in the chassis leg fixing them firmly in place.
