Back to re-address the topic, this time with some solid information. While I've gotten some pretty varied opinions on this subject here, unfortunately no one with any real knowledge on the question has joined in with the discussion. With nothing forthcoming from this web site, I turned to my engineering contacts, Permatex, and Locktite for some real info.
The primary problems faced when using steel with aluminum (especially stainless steel) are galling and galvanic corrosion. Gallling is where a harder metal comes into frictional contact with a softer one (as in the case of SS and aluminum), which results in the softer metal smearing onto the harder stuff. To eliminate this, a lubricant is all that is required and, in fact, the old school bikers would use grease, oil, or even wax on their bolts in an attempt to alleviate this problem . The condition is especially prominent in areas of high temperature, where most lubricants simply break down and evaporate; and the different expansion rates between different materials in high temp conditions comes into play here, as well. All of these factors can cause things like steel spark plugs and exhaust studs to seize tight in an aluminum head, and their removal can result in damage to the threads they're screwed in to as the aluminum smeared onto the steel is distorted or even removed with the bolt.
To combat this, Loctite, Permatex and other anti-seize manufacturers have developed an anti-seize containing copper in suspension. Copper is well known for its excellent high temp properties; that's why it's used in in applications ranging from colliders and tokamaks to back-country stills to the bottoms of your (steel, not aluminum) cookware. It absorbs heat evenly and dissipates it just as well, all the while maintaining it's own molecular properties. In paste form, and under pressure, the particles bind together to create a lubricative barrier between adjoining metallic faces that remains soft without melting or otherwise dissipating. And its excellent electrical conductivity is especially important in areas like the sparkplugs, where a good path between the head and plug body is absolutely essential.
However, use of copper outside of high temp conditions can result in problems due to galvanic corrosion. An in-depth investigation of galvanic corrosion would take up more space than I'd care to expend, so I'll try to thumbnail the whole problem.
The Galvanic Series (abridged)Galvanic Corrosion occurs when two dis-similar metals are in direct contact with each other and an electrolyte (water, acid, etc) is present. The electrolyte can begin pulling electrons off each material (oxidation), which sets up an electrical current between the two metals, creating a galvanic "cell" with the more stable ("noble", or more resistant to corrosion) metal acting as a cathode, and the more "active" (or more easily corroded) metal serving as the anode. This acts to strip electrons from the active material at a greater rate than would be normal in non-galvanic conditions; i.e. two similar metals in contact with a corrosive agent. If the two metals are far enough apart on the galvanic chart (based on the electrical potentials between the two, or "corrosion potentials"), the active metal will corrode at an exponentially greater rate than the more noble metal will; in fact, it can result in no corrosion of the noble material at all, while the active metal is consumed. And in conditions where the electrolyte contains a lot of free electrons, like near the ocean or in cities with a lot of CO2 and sulfide pollution, the condition can become a real problem. It has been the bane of ship builders since the dawn of copper-bottomed boats, and of mechanical engineers since the first time they tried to attach their fancy new iron axe blades to their old copper handles, and they've all been looking for solutions ever since.
To help eliminate this, engineers are encouraged, first of all, to avoid mixing dissimilar materials. Since this is pretty near impossible, their next form of defense is a good offense; or rather, it's more properly a case of "if you can't beat 'em, join 'em". They use the galvanic series to their advantage, and kind of trick mother nature with distraction by introducing a third metal into the mix; one that is even less noble than the first two. It acts as a "sacrificial anode" which is consumed by the electrolyte in deference to the other two more noble (but dissimilar) metals, slowing their corrosion (kind of like having a bear invade your picnic and tossing him the strawberry cobbler in order to save the pork roast and the green bean casserole). This is a pretty common practice; in fact, your hot water heater has a "sacrificial anode" made of either magnesium or aluminum that saves the steel tank from corrosion due to galvanic potentials between the tank and the minerals in the water.
Honda's engineers have addressed this by plating all of their steel fasteners, and a considerable number of other steel parts, with zinc. Zinc is right at the top of the galvanic chart, being less noble than either steel or aluminum, and cheerfully gives up its life for the greater good. The use of zinc, in fact, is an industry-wide practice, and you see it everywhere; plated, it has a nice, shiny metallic blue tone. Cadmium. with its distinctive mottled red/green/gold coloration, has been used for the same purposes; and though it's being phased out due to its high toxicity, it and zinc are commonly used world-wide as barriers against galvanic and other forms of corrosion.
Another approach is to introduce the "sacrificial" material in the form of a corrosion-inhibiting paste, which is where anti-seize comes in. It works to stop corrosion in many different ways: by introducing a less noble metal into the galvanic mix; by forming a barrier that, at least temporarily, keeps water (electrolyte) from entering into the system (most are at least 50% grease, which acts to eliminate galling, as well); and by creating an anaerobic condition that inhibits oxidation. For most of our case bolts, where steel and aluminum come in contact, Locktite (and others) recommend an anti-seize containing (surprise, surprise) zinc . Its use reduces the corrosion potential between the more noble steel and the more active aluminum, and it's pretty much a requirement when stainless bolts are used instead of the standard zinc plated fasteners.
I know there are a few who swear by the copper-based anti-seize (some quite rabidly, actually) and use it for everything they can lay their hands on; not only that, they've been using the stuff for (insert number of years/decades/eons here) without a single problem. And that may be the way things appear to them. Copper and aluminum are pretty widely separated on the galvanic table, and in galvanic conditions aluminum always loses out to copper. That's why you don't mix copper and aluminum wiring in your house, for example. Honda uses copper on the bike primarily as crush washers (not including electrical switches), and in all cases except one, they are plated to prevent corrosion. The exception is the exhaust port crush rings, or exhaust packings. Four copper rings that fit between the head and the exhaust pipes, copper being utilized here because of its high-temp properties, as already discussed. These are protected by the high temp paint on the head, which eliminates one condition necessary for galvanic corrosion: direct metal to metal contact. In addition, the high temperature at this location (as at the plugs) and the angle of the head make it difficult for an electrolyte to form; and, as the washers are tightly formed into the port by crushing, it's fairly anaerobic in the contact point between the washers and the head. All this doesn't eliminate corrosion altogether; the next time you replace your pipes, check the contact perimeter on the face of the exhaust port and I can almost guarantee you'll find a faint circle etched into the aluminum that delineates the outside edge of the copper/aluminum contact patch.
The thing to keep in mind, here, is that all of this is based on events occurring at the molecular level while in real life things aren't so black and white. Metals alloyed, formed, forged and cast into widgets can react differently in structure than their molecular components would indicate, some becoming more noble in solid form (chromium is a good example), others less so, and the whole galvanic table kind of squishes together so that reactions aren't quite as extreme as you might expect them to be. Still, the strictures regarding dissimilar metals need to be heeded, so while using copper anti-seize along with your stainless steel case bolts won't necessarily cause the engine to crumble before your eyes, it isn't how Honda designed things, nor is it the best thing you could do for your bike in the long run; and since I want my bike to outlast me, I won't be using copper anywhere other than the spark plugs and exhaust bolts, which is where its use is recommended. If Permatex, Locktite and all the other anti-seize makers had wanted the copper stuff to be used as an ubiquitous cure-all, they wouldn't have gone to the effort and expense of formulating other versions, especially those aimed specifically at steel/aluminum interactions.
