Re: Anti-seize: copper or aluminum-based? Mixing OK?
The answer is yes and no. An anti seize is composed of a lubricant plus metal flakes. The flakes are there principally to avoid dissimilar metal reactions between the two mating surfaces. The choice between copper and aluminum is determined by the metals involved in the mating, plus by temperature. , and the anti seize function is still delivered by either metal for basically any metal ever encountered in cycling. If we had pure beryllium bikes mated with lithium bottom bracket shells, we might have an issue. But for ti, aluminum, and various steels, the type of flake isn't really all that important. Copper costs slightly more and is slightly more effective in preventing galling, which is a different phenomenon from dissimilar metal corrosion, which is why it's more commonly used in ti frames. In practice, this may be because the copper flakes are more durable and preserve some separation between the mating surfaces and allow that gap to contain some grease, which has nothing to do with chemical reactivity and is purely a mechanical matter. It's an issue that becomes relevant in high speed high temperature titanium applications such as aircraft engines, but not really in bikes. If you see both on the shelf, get the copper version. And I'd say that more because the aluminum seems to dry out a little faster, which leads us to the opposing issue in this discussion.
The other part of anti seize is the grease base used. Now there are lots of different kinds of grease used in copper anti seize, driven by high temperature needs, speed issues, and so on. You're more likely to run into incompatibility in the greases used. They can be natural, synthetic, and all kinds of things in between. Just because they are greases doesn't mean that they will mix and live hospitably together. So practically speaking, it's the grease in the anti seize that creates the biggest issue. And thus my recommendation would be to clean one out completely before using the other. And I'd say the same thing with regard to replacing copper anti seize with more copper anti seize. Unless they came from the same can, clean one out before introducing the other one. One might have a very different grease base from the next.