Fastener finishes

[Tom L.]

I am currently putting the rear suspension on my '72 back together and am using the "Corvette chassis restoration guide" to help with the different finishes on the fasteners. The table in the back of the book indicates that the fastener finishes are either "plated" or "black" but doesn't indicate what type of blackening or plating that was used. Can someone elaborate on this? Thanks!!

[Chuck S.]

Re: Fastener finishes

What the author means is most fasteners are either zinc plated or black phosphate. Some fasteners can be black oxide, but rarely on the chassis.

[Edward J.]

Re: Fastener finishes

Lynn, when I did my 72 the chassis fastners were mostly blk. phos., there were some zinc plated bolts that hold the fuel tank brace and straps, and the antenna ground strap retainers were zinc. I did last year see some pic.s of a 300 mile ZR1 car. from one of the members that had posted on the TDB and there were some zinc bolts clearly mount on the rear bumpers w/ blk. phos. washers . I believe that what ever was in the buckets that day is what were used.judging wise I would bet that either would be correct.

[Tom L.]

Re: Fastener finishes

Pehaps a silly question but aren't there different shades of zinc plating? Is the plating you refer to similar to the plating found on common grade 5 hardware store bolts? There is no plating left on my fasteners to refer to, time and the environment got the best of them.Thanks!!

[Chuck S.]

Re: Fastener finishes

Pehaps a silly question but aren't there different shades of zinc plating? Is the plating you refer to similar to the plating found on common grade 5 hardware store bolts? There is no plating left on my fasteners to refer to, time and the environment got the best of them.Thanks!!

Yes, I know of at least two different zinc finishes...I believe the difference is in the final treatment rather than the zinc plating. If I recall correctly, the most common zinc plate is "blue" or "clear" zinc...this is what you see on HD hardware. It's the most durable and most common. I don't remember the other zinc finish name, but I think it's just the plated zinc without the final bath.

In fact, I said zinc plated, but cadmium plating was also used, sometimes interchangeably. Cadmium is more durable than zinc, but there are fewer cadmium platers in the US, and the plating has become more expensive as environmental rules have become stricter. The fact of the matter is visual appearance of the two plating methods is virtually undectable; anyone that thinks they can see the difference is probably observing the difference in the way the metal was prepped or finished before plating.

Since we don't have the drawings to see what finish was originally specified, and cadmium plating is more toxic and expensive, and the difference is undetectable anyway...I just used zinc for plated fasteners. Surface prep IS important to final appearance...bead blasting will give you a matte finish that some associate with cadmium; wire brushing or acid cleaning will give you the slick, shiny finish that you typically see on HD (and some original GM) fasteners.

Edit: There are not many zinc fasteners and other hardware items on the chassis. If there is no remaining evidence of the original finish, you will have to take each fastener on a case by case basis. Check for overall condition as well as rust...sometimes, even though a fastener is rusted, deterioation will be far less for fastener that was originally zinc plated than most that are phosphate finished. Check the sheltered area at the head and shank junction, and the first thread under the head; if there's any zinc left, that's where you will see it. Unfortunately, location/exposure also plays an important role in this evaluation.

[Edward J.]

Re: Fastener finishes

Lynn, Most of the bolts you find in the hardware stores are cadmium plated, Zinc plating may vary slightly. cad. plating is close to Zinc. but has a flat silver finish.

[John H.]

Re: Fastener finishes

Lynn, Most of the bolts you find in the hardware stores are cadmium plated, Zinc plating may vary slightly. cad. plating is close to Zinc. but has a flat silver finish.

Ed -

I'd question that. In addition to being FAR more expensive to cadmium-plate than to zinc-plate due to EPA heavy-metal disposal requirements, cadmium is a known carcinogen by skin contact, and was eliminated from all automotive applications across the industry many years ago as a result. You don't even want to handle cadmium-plated fasteners.

About the only place you'll find cadmium-plated fasteners (or parts) these days is in the aviation industry, and they're VERY expensive.

[Michael G.]

Re: Fastener finishes

John is correct. GM originally utilized cadmium because it offered a rare combination of superior corrosion performance, consistent, low-friction, and conductivity. In the late seventies other finishes became available that offered better corrosion performance as well as equally consistent friction.

By 1980 cadmium had been largely eliminated by GM in all fastener application except prevailing torque (lock) nuts. Even those changed to other finishes by 1990.

Today, due to the dangers of handling cadmium, almost all platers have stopped plating with cadmium. No hardware store fasteners are cadmium plated. Most are either zinc or a paint dip-spin type coating.

Regarding zinc, GM has used two basic processes for applying zinc to the surface of a fastener: 1) the most common, electroplating and 2) "mechanical" zinc, which is a process where the fastener is tumbled in zinc and becomes coated with essentially powder zinc.

Electroplated zinc is smooth and shiny. It may be tinted many colors by application of a chromate sealer. Both the zinc thickness and the type of chromate affect corrosion performance. Mechanical zinc is dull gray and rough looking and used infrequently, mostly on high hardness bolts to avoid hydrogen embrittlement.

[Edward J.]

Re: Fastener finishes

John is correct. GM originally utilized cadmium because it offered a rare combination of superior corrosion performance, consistent, low-friction, and conductivity. In the late seventies other finishes became available that offered better corrosion performance as well as equally consistent friction.

By 1980 cadmium had been largely eliminated by GM in all fastener application except prevailing torque (lock) nuts. Even those changed to other finishes by 1990.

Today, due to the dangers of handling cadmium, almost all platers have stopped plating with cadmium. No hardware store fasteners are cadmium plated. Most are either zinc or a paint dip-spin type coating.

Regarding zinc, GM has used two basic processes for applying zinc to the surface of a fastener: 1) the most common, electroplating and 2) "mechanical" zinc, which is a process where the fastener is tumbled in zinc and becomes coated with essentially powder zinc.

Electroplated zinc is smooth and shiny. It may be tinted many colors by application of a chromate sealer. Both the zinc thickness and the type of chromate affect corrosion performance. Mechanical zinc is dull gray and rough looking and used infrequently, mostly on high hardness bolts to avoid hydrogen embrittlement.

Thanks for for your input, I always thought the the shiny plating was cadmium , and that the zinc was the flat silver finish.I learned something new. today with the plating.

[Tracy C.]

Re: Fastener finishes

I am currently putting the rear suspension on my '72 back together and am using the "Corvette chassis restoration guide" to help with the different finishes on the fasteners. The table in the back of the book indicates that the fastener finishes are either "plated" or "black" but doesn't indicate what type of blackening or plating that was used. Can someone elaborate on this? Thanks!!

If you can get past the fact that this has a "Ford" based genesis...the link below might help some in recognizing fastener finishes..

tc

http://429mustangcougarinfo.50megs.com/new_page_26.htm

[William D.]

Re: Fastener finishes

Nice link with great info. Thanks, Tracy.
Bill

[Ronald L.]

Re: Fastener finishes

Fasteners finishes are not any real secret, sort of. A couple years ago a member here posted a page - a wonderful matrix, but unfortunately very low resolution...

The horizontal axis was bolt size, and the vertical axis had the finish type, plain, CD ZN, Phos, Chrome. In the matrix are the GM part numbers.

You get your part number from your AIM and look at the matrix to find the finish.

That is what I call the Gold Standard.

When I restored a 66 in 1980, that page was in a book on my desk so reapplying the correct finish was pretty easy. Unfortunately that book stayed on my desk when I left that job.

[Alan S.]

Re: Fastener finishes

Hi Ron,
Can you offer any clues as to what to 'search' for in order to find that post?
Regards,
Alan

[Chuck S.]

Re: Fastener finishes

I also seem to have missed that thread. I'd be very interested in finding that thread as well...some fragment of the original poster's name or an approximate time period would be helpful.

Since there are probably thousands of GM fastener part numbers, I can see where resolution would have been a problem, even for one year class...for all GM fastener part numbers, it would seem a book of matrices would be needed.

[Ronald L.]

Re: Fastener finishes

Actually all fasteners 1" and less were on page one, 1 -2" or so on the next page, etc.

It is proving problematic to find the thread as the post was by Franz and he is no longer active on this forum...