C3 zinc backing plates

[Rob C.]

Could any one tell me or have a good color picture of a unpolished zinc backing plate? I would like to do mine over. Thank You.

[Chuck S.]

Re: C3 zinc backing plates

Rob, I don't have a picture, but I can describe the finish.

Original zinc backing plates are actually a galvanized finish rather than zinc plated. If you look closely, the finish will have the mottled look of galvanized steel, which is due to large zinc "crystals" that form when the molten zinc cools. If you have ever observed galvanized steel roofing material, the appearance is the same; the finish turns from bright silver to dull gray as the zinc ages, but the mottled appearance remains visible.

As I recall, my originals were actually rusty on the die cut edges leading me to believe that the backing plates were fabricated from bulk galvanized sheet stock. I don't think you have the capability to re-galvanize yours, but clear or blue zinc plating will be close. Baer's Corvettes also sells the zinc reproductions, and I believe they are accurate.

Be advised that the galvanized backing plates were only used up to 70 or 71, I forget which year. For 72 and later, the backing plates were plated zinc with a gold dichromate wash. For many years, all you could get were the GM service replacemments which were also gold dichromate.

[Gregory G.]

Re: C3 zinc backing plates

Chuck,

My '70 has the zinc backing plates, 3 of them have stamped part numbers on them .Part numbered plates are they service replacements or production parts?

Greg

[Joe L.]

Re: C3 zinc backing plates

Chuck and Rob-----

I agree. I believe that many, if not all, disc brake backing plates used in the 69-71 period, if not earlier and later, were manufactured of pre-punch galvanized material. I'm 100% certain that the original ones on my 1969 were. However, I believe that this applies to the REAR shields only. I've never seen a FRONT shield that evidenced this, but there certainly may have been some.

The original front shields on my 1969 were zinc plated. In fact, they were not only zinc plated, but they also had the irridite (gold) over-plate. So, the front and rear shields on my car did not originally "match". The front were "gold" and the rear were "silver". Generally, I think that it's "conventional wisdom" that the irridite over-plate shields did not come along until later C3's. However, the specification for the over-plate was on the drawing for the part from the beginning and, at least some, I believe were delivered that way to PRODUCTION earlier than many folks think. I know that at least ONE 1969 got them, anyway.

Anyway, why the difference between the plating for the front shields (zinc, with or without overplate) and the rear (pre-punch galvanized material on some or all)? Well, I believe it's due to the different "genesis" of the front and rear shields. The front shields were a Chevrolet-spec'd item and carried Chevrolet part numbers. The rear shields were a Delco-Moraine spec'd item and carried Delco-Moraine part numbers. Consequently, the whole design and manufacturing process, including suppliers, was likely different even though we think of the front and rear shields as a "matched set".

[Chuck S.]

Re: C3 zinc backing plates

Greg, my opinion is that they are original.

If they are an aged dull gray, they are most likey the ones that came on the car. I understand the Baer's repros are very good, and that may include stamping with the proper part number...they could be their repros if they are bright. Backing plates are not a wearing part, so in my opinion, finding originals on your car should not be too surprising.

I doubt that they are service replacement backing plates. GM typically changed the service parts to the latest iteration of the design...since the backing plates changed in 72 (I think), I would expect service replacements sold after the early 70s to be gold dichromate.

Joe, you are probably right about the fronts being plated and the rears being galvanized. I thought it was the fronts that I observed the galvanized finish on, but thinking about it, I did have the fronts zinc plated...not like me to compromise on a finish if I can avoid it (Baer's had NOS backing plates at the time). I suspect your gold dichromate front backing plates would fall into one of those "owner has to prove it" items in judging...I had not heard of 69s having gold dichromate, but I learn at least one new thing everyday.

[Joe L.]

Re: C3 zinc backing plates

Chuck----

Oh, the original finish of my backing plates is a moot point, now. I had the both the fronts and the rears re-plated in clear zinc. So, now they're "original". Actually, though, that's not the reason I did it. The rears were getting a bit "rough" looking and I didn't like that the fronts and rears didn't match. So, now they're all exactly the same clear zinc. Coincidentally, the same way that they're "supposed to be".

[Terry M.]

Re: C3 zinc backing plates

There is a very low mile 1972 which received a Bow Tie award at Hershey. The rear backing plates are very obviously die cut galvanized sheet. The ink stamped manufacturers' logo was visible on one of the backing plates -- that is the manufacturer of the sheet stock. I have a photo of that ink stamp if someone can host it.
FWIW: The front shoelds on this car are silver also. May of 1972 build.

[Joe L.]

Re: C3 zinc backing plates

Terry-----

Were the fronts pre-punch galvanized, too, or were they clear (silver) zinc plated?

[Terry M.]

Re: C3 zinc backing plates

I couldn't tell the method of manufacturer for the fronts. I did look for the unplated edges on the rear. I was able to see those, but that feature didn't photograph well. However, based on the ink stamp, and its distortion, I believe any reasonable person would be convinced of the method of manufacture of the rears.

[Greg L.]

Re: C3 zinc backing plates

I've been following this thread and I just clued in to the fact that I told my plater to cad plate my backing plates last week! I don't know if they are done yet but is cad and zinc close enough that judges don't worry about it or should I get him to zinc plate them. I'm not sure where I got the idea that they should be cad....maybe someone had said that unpolished zinc appears like cad?....