Rear bumper hardware again!

[Bill L.]

Re: Rear bumper hardware again!

HI Gary,

The cad comment was just a general comment as that most consider it a premium finish. I understand why they stopped using it.

I am just a little surprised that the flat washer was zinc plated when previous posts indicate either plain steel or zinc phosphate as typical finishes.

I am also curious how the finish is determined via the part number.

Thanks again,

Bill

[William C.]

Re: Rear bumper hardware again!

Part number only takes you to the print, which if in your possession will spell out finishes/coatings, along with a list of changes/revisions and dates. In some cases multiple part numbers on similar parts will be contained in a "gang" drawing carrying multiple similar parts. That drewing will have it's own part number different from the part you are looking for which must then be used to dig out the details for a specific item.

[Bill L.]

Re: Rear bumper hardware again!

Part number only takes you to the print, which if in your possession will spell out finishes/coatings, along with a list of changes/revisions and dates. In some cases multiple part numbers on similar parts will be contained in a "gang" drawing carrying multiple similar parts. That drewing will have it's own part number different from the part you are looking for which must then be used to dig out the details for a specific item.

What is the likelihood that the finish might vary from the print?

[Bill L.]

Re: Rear bumper hardware again!

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

Well while I am at it. What do the prints show the finish to be for pn-9419077 and 9419075? Do you have their description too. I notice that one of the two has an unthreaded portion while the other is threaded for the entire length. I know where I removed them from but I was not the first to disassemble the rear bumpers and brackets. Headmarks are what should expected. (A or M with 3 lines) I know there was a change somewhere as midyear bolts I have looked at are not grade 5 with the 3 lines.

Thanks, Bill

[William C.]

Re: Rear bumper hardware again!

Slim and none, and I think slim just left the building. GM was a very part number centric operation.

[Gary B.]

Re: Rear bumper hardware again!

Bill,

I don't have acceess to any GM engineering drawings, so in general I can't help you with those questions. The reason I asked for part numbers is that sometime in the past year or so, someone posted a scan of a page from one of the GM parts catalogs that had a list of about 20 part numbers for various washers of different sizes and types, along with the finish. From that list one can recognizes an association between the first 3 digits of the part number and the finish. For example, parts numbers starting with 103xxx had a plain finish. Whereas, 120xxx were cad or zinc. I have an NOS example of one of the 103xxx family of lock washers and it is indeed plain finish, as heat treated, and quite a dark gray. Of course, this is just a theory on my part, but I've yet to see any proof that my theory does not hold.

Regarding your question about finishes deviating from what is specified on the GM engineering drawing (again, I don't have access to any prints), I can only speculate once again that such a deviation is unlikely. Perhaps Bill Clupper or John Hinckley or someone else could speak to that question.

Gary

[Bill L.]

Re: Rear bumper hardware again!

Slim and none, and I think slim just left the building. GM was a very part number centric operation.

Thanks! I am almost straight on all this with new notes in my AIM.

Bill

[Gary B.]

Re: Rear bumper hardware again!

...zinc phosphate.

Thanks, Bill

Bill,

One last question. What do you mean by "zinc phosphate"? One often hears of a zinc finish described with modifiers "clear" or "silver" zinc, but I've never seen the term phosphate associated with the zinc finish.

Gary

[Bill L.]

Re: Rear bumper hardware again!

Bill,

One last question. What do you mean by "zinc phosphate"? One often hears of a zinc finish described with modifiers "clear" or "silver" zinc, but I've never seen the term phosphate associated with the zinc finish.


Gary

I usually refer to zinc as the silver/blue finish versus the zinc phosphate which is a gray to dark gray phosphate that has little sparkles in it.

Bill

[Gary B.]

Re: Rear bumper hardware again!

I usually refer to zinc as the silver/blue finish versus the zinc phosphate which is a gray to dark gray phosphate that has little sparkles in it.

Bill

Bill,

In all my references to zinc, I'm referring silver zinc. For the dark grey finish with the little sparkles, I always use the term black phosphate, which I think is what many others use as well. That avoids the possible confusion of people (like me!) of just focusing on the zinc term and having some people think of a bright silver finish and others thinking of a dark gray finish.

Gary

[John H.]

Re: Rear bumper hardware again!

I usually refer to zinc as the silver/blue finish versus the zinc phosphate which is a gray to dark gray phosphate that has little sparkles in it.

Bill

Bill -

The "phosphate" treatment used on GM fasteners that produced the dark finish with the "sparkles" in it was manganese phosphate, with an oil coating, commonly noted on GM drawings as "phosphate and oil".

[Ronald L.]

Re: Rear bumper hardware again!

Guys, if it was black or dark gray it was phosphate, I looked at all the fastener prints the prints in '79 and replated or re phosed all the originals bolts washers and nuts when I restored my first 66.

As Bill mentions, all the cad talk is myth, it was super toxic and had been eliminated except in very specific applications where nothing else would work...bottom line -it was all zinc plated. Today, if you get stuff plated, as from the 70's on they can put a chromate clear, yellow and other corrosion resistance extenders on - non of that was original to the C2 era.
Tin has zero corrosion resistance. There are also members in NCRS with the GM standards book on their book shelves that have all these same specs, a wealth of information that book is, just they don't normally share that info.