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Thanks for comments guys. I understand the bore is FRM material and also that the Honda workshop manual seems to steer away from honing unless there is reason to. Given the bores are otherwise in very good shape I am not going to hone them. In light of comments above I now believe the marks are from a chemical reaction of some sort or another. I used some 00 grade steel wool to very, very lightly buff the area and did seem to bring back some of the factory bore surface. In the end though I am going to leave bores as they are because I think I'll end up in more trouble than it's worth.
Correct, Honda wanted to create a lighter and more compact engine. The paper states that when aluminum-silicon alloy is used, it is difficult to achieve the proper crystallization of the silicon and the material requires an etching process, which drives up costs. FRM was developed so that ordinary aluminum could be used while maintaining proper wear resistance and bore deformation parameters.
To the OP, another option is to have the walls finish honed to see if that stain comes out. Not knowing what chemical damage may be present is a gamble on longevity. For all we know that thing could be microscopically peeling off.
There is that service bulletin from Honda circa 2001 floating around which states that discoloration or flaking is grounds for block replacement. Please someone correct me if my memory fails me.
Like I said that's just aluminum oxide. Yes FRM is not silicon, however the FRM is still embedded in the aluminum. Exposure is not the same as etching. It consists of using a felt pad and very fine SiC paste. What this does is peel a .000002" layer of aluminum off the bore and leaves the FRM exposed. Which is why they call it an exposure step. It doesn't matter what substrate is being used. Silicon, Fiber reinforced metal, ceramic fibers, etc, the process of honing is the same.
The whole idea behind the FRM development was to avoid the extra expense and complications of etching and exposure steps. The research paper explicitly states this. The block does require special machining (diamond stones for non-ferrous metals) but, it does not require special processing such as etching or exposure. The repair manual also does not state that special processing is required after honing.
The carbon fiber is there for re-inforcement only; it is not the wear surface. Remember that either you have an aluminum piston riding on an iron liner, or a iron plated piston in an aluminum bore. Our pistons are iron plated, which makes it an "FRM friendly piston."
This is the reason why we can't run aluminum pistons on our aluminum bores, as similar materials cannot wear together properly. The way to make the OEM piston work is by iron plating it.
FRM pistons are still aluminum the difference is the heat expansion rate. FRM doesn't expand like iron liners, as they're aluminum sleeves. It's got nothing to do with plating. S2k pistons are not magnetic.
This is the reason why we can't run aluminum pistons on our aluminum bores, as similar materials cannot wear together properly. The way to make the OEM piston work is by iron plating it.
The rings are what touches/rides the cylinder bores, not the pistons. If you have piston contact with the cylinder bores your on barrowed time. I've never heard of a iron plated aluminum piston. Our motor certainly doesn't have one.
FRM pistons are still aluminum the difference is the heat expansion rate. FRM doesn't expand like iron liners, as they're aluminum sleeves. It's got nothing to do with plating. S2k pistons are not magnetic.
What I stated was that the piston was iron PLATED, not that the piston is made of iron or magnetic. The pistons require an iron plating otherwise they would scuff the aluminum. If the pistons are not iron plated, then how can you explain this?
