Quote:
Originally Posted by DeadheadNM
I seem to recall that zinc phosphate produced a glossy finish whereas manganese phosphate produced a flat, matte black finish. I believe the latter was typically used for GM fasteners etc. Is it possible your batches are a mix of zinc and manganese?
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You could be right, and I once believed so, but I'm more inclined to believe most were zinc phosphate. Here's a site ddentifying the differences, I put their info. into captions. I looked at some of the old bolts on my un-restored C20, where I can get past the rust I can read the markings clearly. Still doesn't mean the manganese didn't wear off.
http://429mustangcougarinfo.50megs.com/new_page_26.htm
Come to think of it, I have some NOS bolts, I'll pull them out later, take a picture.