FYI the metalurgy is the same as the Vega, but Mercedes used a slow low pressure casting process rather than the high pressure die casting process used by GM.

Bore finishing initially consisted of boring and honing in the conventional sense. The OEMs then used an alkyline solution with electrical potential and current applied to etch away about .0005" of surface aluminum leaving the pure silicon wear surface.

In the field this etching process is replaced by the "silicon lap" to remove surface aluminum. You must also use pistons with the proper plating to be compabible with the bore surface, which are available off-the-shelf for Mercs. I've found shops that work on Merc aluminum blocks, and they can do proper bore refinishing for Cosworth Vega blocks, but there are no off-the-shelf pistons with the proper plating, so the most expedicious and least risky path for CV owners is to install dry iron sleeves in the bores and install semi-custom forged aluminum pistons that are made in batches from time to time.

I had the head of my Cosworth Vega off for a refresh last year at 70K miles and could not measure ANY bore wear/taper using both dial bore gages and inside gages and micrometers. I was quite impressed.

You mention silicon dioxide as opposed to pure elemental silicon, which is my understanding of what the wear surface is. In any event, inspecting these bores with a borescope is interesting. The surface is a mottled gray color with no evidence of the final hone, and you can see small darker gray splotches homogeneously distributed in the field of view. These are the silicon particles that form the actual wear surface.

Duke

Duke