I'm relatively new to the discussion board and came across several messages from late August 2002 relating to the finish/color of the dust shields. With appologies for being so late here's a bit more.
Joe Tripoli's chassis book has portions of the engineering drawings for several of the parts being discussed including front and rear shields. All call for the following finish: Chromate Treated Zinc Plating - GM 4252-M, Code 20, Plus 4468-M (some additionally state: Finish to be as taken from the plating bath). I don't see any reference to clear or gold chromate and have always believed that it was optional to the supplier. Gold chromate is slightly more expensive and less common than clear so I've assumed that the clear was most often used, though either was acceptable to GM. This is consistent with the observed finishes. For example the caliper support brackets came through gold while the dust shields came through clear. Later on, either the spec was modified or another supplier used gold chromate and the service parts were different. I also have never seen gold original dust shields but the spec seems to allow for them so possibly they were gold on occassion. Someone (Terry?) with access to the GM spec might have more insight.
It's my impression that pre-punch plating is normally restricted to material with a fairly robust finish (ex. dip galvanizd steel) or where there isn't much deformation. Otherwise the stretching/rubbing in the dies will damage the finish. Strictly speaking, the edges of a pre-punch plated part do not meet the finish spec and I'm surprised that the drawings do not include a note allowing for that deviation, if it was permitted. The only difference in the finished part would be that the edges would start to rust well before the other surfaces (could also happen in areas of high deformation). Is this what Joe is seeing? I'm surprised that such distinctions can be made on 30+ year old parts.
Iridite is a 50 year old trade name for gold chromate that is currently owned by Whitco Chemical. It's tended to enter mainstream usage ala scotch tape but I would not expect any GM spec to use it. The correct term is chromate conversion finish, which is shortened to chromate. The basic technique is a dip in chromic acid which produces a clear finish consisting of a very thin layer of ZnCr2O4. Additives are used to alter the pH and change the finish to ZnCrO4, which has a gold color (still other additives produce the olive drab finish on carburetors). I believe that almost all zinc/cad plated parts including fasteners were chromated though most people don't realize that it's there unless it's gold chromate.
Finally, I'm used to seeing the Delco split circle logo on original rear shields but not on most service parts. However I know of a one owner Bowtie '72 with no split circle on one side. He's compulsive enough that I'm inclined to believe him that it never was changed.
Joe Tripoli's chassis book has portions of the engineering drawings for several of the parts being discussed including front and rear shields. All call for the following finish: Chromate Treated Zinc Plating - GM 4252-M, Code 20, Plus 4468-M (some additionally state: Finish to be as taken from the plating bath). I don't see any reference to clear or gold chromate and have always believed that it was optional to the supplier. Gold chromate is slightly more expensive and less common than clear so I've assumed that the clear was most often used, though either was acceptable to GM. This is consistent with the observed finishes. For example the caliper support brackets came through gold while the dust shields came through clear. Later on, either the spec was modified or another supplier used gold chromate and the service parts were different. I also have never seen gold original dust shields but the spec seems to allow for them so possibly they were gold on occassion. Someone (Terry?) with access to the GM spec might have more insight.
It's my impression that pre-punch plating is normally restricted to material with a fairly robust finish (ex. dip galvanizd steel) or where there isn't much deformation. Otherwise the stretching/rubbing in the dies will damage the finish. Strictly speaking, the edges of a pre-punch plated part do not meet the finish spec and I'm surprised that the drawings do not include a note allowing for that deviation, if it was permitted. The only difference in the finished part would be that the edges would start to rust well before the other surfaces (could also happen in areas of high deformation). Is this what Joe is seeing? I'm surprised that such distinctions can be made on 30+ year old parts.
Iridite is a 50 year old trade name for gold chromate that is currently owned by Whitco Chemical. It's tended to enter mainstream usage ala scotch tape but I would not expect any GM spec to use it. The correct term is chromate conversion finish, which is shortened to chromate. The basic technique is a dip in chromic acid which produces a clear finish consisting of a very thin layer of ZnCr2O4. Additives are used to alter the pH and change the finish to ZnCrO4, which has a gold color (still other additives produce the olive drab finish on carburetors). I believe that almost all zinc/cad plated parts including fasteners were chromated though most people don't realize that it's there unless it's gold chromate.
Finally, I'm used to seeing the Delco split circle logo on original rear shields but not on most service parts. However I know of a one owner Bowtie '72 with no split circle on one side. He's compulsive enough that I'm inclined to believe him that it never was changed.
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