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I bought a second-hand bike. It has linear pull brakes. There is a tubular mount made of steel with a male thread which screws into the rear triangle of the bike's aluminum frame. Each brake is secured to such a mount with a hex-socket cap screw that threads into the mount's female-threaded tube.

There was orange loctite on that hex socket cap screw, which caused the entire mount to spin in the frame when the screw was turned, rather than the screw coming out of the mount.

Once I got the entire brake assembly out of the frame I was able to break the loctite bond by holding the mount on its (very skinny) wrench flats and then turning the socket cap screw CC.

Question: Should I put some Loctite (blue? orange?) on the mount itself so it is secured in the frame? Are there any issues when dissimilar metals, steel and aluminum, are bonded with Loctite?

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The part that screws into the frame is a removable cantilever boss. It's a good idea to use threadlocker on them as a precaution. Usually a higher strength one that's still seperable if needed is chosen, but medium/blue would be fine too. They should be torqued pretty heavily when installing. When they come loose, odds are it's because they were under-torqued to begin with. One approach is to pick out the old threadlocker and apply it again fresh so the old stuff isn't adding a bunch of friction to disrupt your torque feel, which would otherwise need to be accounted for.

As far as bikes are concerned, Loctite is pretty universally good at preventing corrosion and other issues between dissimilar metals in all the places you'd want it. It doesn't do much or anything to prevent galling, so in the few spots in bikes where that's a high concern you might choose something else.

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  • Loctite is applied to surfaces in contact to prevent movement. Galling occurs when surfaces move relative to each other with insufficient lubrication. When may galling be relevant to loctitet surfaces? In other words which are those spots where loctite ought not be used?
    – gschenk
    Dec 15, 2018 at 11:11
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    Stainless/stainless, ti/stainless, and ti/ti are all situations where I tend to go for anti-seize over anything else as a way of doing everything I can to prevent galling and other issues. I don't know that I'd universally say don't use Loctite there, but it provides less lubrication during installation than may be desired.
    – Nathan Knutson
    Dec 15, 2018 at 21:40

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