Rear bumper hardware again!

[Bill L.]

Ok- I have reviewed previous threads and I am still confused, even more so after reading the TIMJG. It says outer bolts are Cadmium. Based on a review of previous threads, at best, they were bright zinc since cadmium was a premium finish and not generally used by GM.

I am confident that all the bumper hardware under the body were phosphated including washers and lock washers. Washers and lock washers may have been bare steel but even that was not for sure so I will phosphate that hardware.

I am also pretty confident upon reviewing the threads that the finish on the washers and lock washers on the outside were either phosphate or bare steel too.

This 70 is an early June build. What is the correct finish for the bumper bolts you can see from the outside? Can it be either bright zinc or phosphate? That is my conclusion based on reviewing previous threads. The hardware on this corvette, as a Texas desert car, has very little rust. Other hardware that is known to have been plated with bright zinc has very clear remnants of the plating and in some cases is very well preserved. The bumper hardware shows no signs of a bright zinc finish like I have seen on other hardware removed like the trailing arm bolts.

Thanks,


Bill

[Bill W.]

Re: Rear bumper hardware again!

When I restored my 70 #2650 I think all outside bolts , nuts & washers were dull cad plated .Including the the door strikers . hidden bolts were black .

[Ron G.]

Re: Rear bumper hardware again!

I would not want to say that every Corvette rear bumper hardware was finished this way, but the majority of Bowtie and Survivors Corvettes that I have had the opportunity to judge or just look at (and believe me there were hundreds of them) had the rear hardware black oxided from 68 thru late 71 production. After that point my observations were noted that the rear hardware was silver cadmium.

[Bill L.]

Re: Rear bumper hardware again!

I would not want to say that every Corvette rear bumper hardware was finished this way, but the majority of Bowtie and Survivors Corvettes that I have had the opportunity to judge or just look at (and believe me there were hundreds of them) had the rear hardware black oxided from 68 thru late 71 production. After that point my observations were noted that the rear hardware was silver cadmium.

Black Oxide or Phosphated? I thought it was generally phosphated. Black Oxide is certainly easy enough to do.


Bill

[Ron G.]

Re: Rear bumper hardware again!

Definately black oxide.

[Gary B.]

Re: Rear bumper hardware again!

Bill,

Do you have the 1970 AIM that lists the GM part numbers for the bumper lock washers and the flat washers? Those part numbers should provide a clue as to the finish. If the lock washers are 103328, then I think they were plain, heat treated, which gave them a very dark gray appearance.

Gary

[Bill L.]

Re: Rear bumper hardware again!

Bill,

Do you have the 1970 AIM that lists the GM part numbers for the bumper lock washers and the flat washers? Those part numbers should provide a clue as to the finish. If the lock washers are 103328, then I think they were plain, heat treated, which gave them a very dark gray appearance.

Gary

Hi Gary,

That is the part number according to the aim. This explains why they are dark gray even after I cleaned them up.

Thanks, Bill

[Gary B.]

Re: Rear bumper hardware again!

Hi Gary,

That is the part number according to the aim. This explains why they are dark gray even after I cleaned them up.

Thanks, Bill

Bill,

Yes indeed! The history of the lock washer for bumper attachment locations is quite interesting. In 1963 and 1964 that lock washer was zinc and it had a regular thickness (GM #120383). Then in '65 that washer changed to plain, but still with regular thickness (GM #103322). Then in mid-'65 it changed to GM #103328, which was plain, but thicker, the so-called hi-collar style. The change from zinc to plain and then to hi-collar suggests an initial problem with strength (a zinc plated washer will be less strong than an unplated washer of the same dimensions), which must have been solved by the eventual switch to the 103328 washer, since that one was used for many years, well into the C3 era.

Gary

[Bill L.]

Re: Rear bumper hardware again!

Quite fascinating Gary. I am constantly amazed at the depth of knowledge here.

Would the flat washer be black oxide, plain steel, or zinc phosphate.

Thanks, Bill

[Gary B.]

Re: Rear bumper hardware again!

Bill,

What's the part number from the '70 AIM for the flat washer that is used at most bumper attachment locations? I know what it is from '63 thru '68, but not after that.

Gary

[Bill L.]

Re: Rear bumper hardware again!

Bill,

What's the part number from the '70 AIM for the flat washer that is used at most bumper attachment locations? I know what it is from '63 thru '68, but not after that.

Gary

pn-9419276

[Gary B.]

Re: Rear bumper hardware again!

Bill,

That flat washer was used back thru 1966. It is zinc plated.

Gary

[William C.]

Re: Rear bumper hardware again!

Cad was both higher cost and by the 1970 timeframe was becoming recognized as a hazardous material for the operators of plating lines and such. I had some responsibilities for the Plating area where I worked (GM facility) and that period caused most of the cad work to be transitioned to other plating, primarily Tin or zinc. If visible, Tin was usually the plating of choice as Zinc lost it's appearance fairly quickly.

[John H.]

Re: Rear bumper hardware again!

I would not want to say that every Corvette rear bumper hardware was finished this way, but the majority of Bowtie and Survivors Corvettes that I have had the opportunity to judge or just look at (and believe me there were hundreds of them) had the rear hardware black oxided from 68 thru late 71 production. After that point my observations were noted that the rear hardware was silver cadmium.

Ron -

I would question the use of black oxide as the finish on under-the-car bumper fasteners; black oxide is the most expensive premium finishing process for any fastener, and I can't imagine a release engineer or fastener engineer spending that kind of premium money specifying black oxide (and getting it past the Finance guys) for bumper nuts and washers that aren't visible.

[Gary B.]

Re: Rear bumper hardware again!

Bill,

If you're referring to the change from zinc/cad plating for the 63-64 lock washers, to plain, as heat treated, for '65 and onward, cost reduction could have been a consideration, but the fact that there was also a switch from the regular lock washer to the presumably more expensive, thicker, hi-collar, lock washer points to a strength consideration as well.

Gary