Finishes For GM Bolts and Nuts

[Joe L.]

Often times we hear of original bolt and nut finishes as being described as phosphate, zinc, or cadmium. However, there were actually many different finishes used for original GM bolts and nuts. Here are examples of a few [none for sale]:

Some GM bolts have what GM specified as a plain finish. Some folks think this is a natural steel finish. However, it's NOT. A plain finish is very similar to what are often described as black phosphate. These finishes, thus, are easily confused and mis-described. The following pictured bolts and nut are all of plain finish. As far as I know none of these were used on a Corvette but they are pictured only to show a typical plain finish. Although it's unimportant, these are, from left to right, GM #9430262, 454867, 9422369, 3799830, 271629, and 14054921:

DSCN3869.jpgDSCN3870.jpgDSCN3855.jpg


The following bolt has a zinc-organic finish. This is a GM #9439920. This finish is sometimes described as "dull zinc" but it's actually different than a pure zinc finish which is usually much brighter.

DSCN3860.jpgDSCN3864.jpg




The following bolt has a phosphate-zinc-organic finish. It's GM #9439920:

DSCN3873.jpgDSCN3874.jpg

The following nut was used on 1965-82 Corvettes. It is a GM #102647 or, possibly, GM #274428 and is of plain finish. It was used for the p/s ram to relay rod, idler arm to relay rod, control valve to pitman arm and relay rod to pitman arm. It was also used for the upper ball joint stud.

DSCN3862.jpgDSCN3867.jpg

The following nut is a GM #9422298. It was used for the idler arm-to-frame bracket bolts. It is of zinc finish:

DSCN3861.jpgDSCN3866.jpg

[Mark F.]

Re: Finishes For GM Bolts and Nuts

Often times we hear of original bolt and nut finishes as being described as phosphate, zinc, or cadmium. However, there were actually many different finishes used for original GM bolts and nuts. Here are examples of a few [none for sale]: Some GM bolts have what GM specified as a plain finish. Some folks think this is a natural steel finish. However, it's NOT. A plain finish is very similar to what are often described as black phosphate. These finishes, thus, are easily confused and mis-described. The following pictured bolts and nut are all of plain finish. As far as I know none of these were used on a Corvette but they are pictured only to show a typical plain finish. Although it's unimportant, these are, from left to right, GM #9430262, 454867, 9422369, 3799830, 271629, and 14054921:

Hi Joe,

Thanks for the informative finish explanations.

I can't put my finger on my reference to it right now, but each and every "finish" GM required also had an accompanying GM engineering spec that prescribed the finish requirements: min/max finish thicknesses; composition; durability; testing specs; etc., Correct?

[Joe L.]

Re: Finishes For GM Bolts and Nuts

Hi Joe,

Thanks for the informative finish explanations.

I can't put my finger on my reference to it right now, but each and every "finish" GM required also had an accompanying GM engineering spec that prescribed the finish requirements: min/max finish thicknesses; composition; durability; testing specs; etc., Correct?

Mark------


Yes.

[Gary C.]

Re: Finishes For GM Bolts and Nuts

Joe,

GRRRREAT info. Don't mean to hijack, but thought this info on parts finishing is kinda related.

Painted parts finish - was told by a retired GM engineer years ago that normally an engineer specified the less cost painted finish for a lot of parts which was "Dip Painted". And that's what the suppliers would get paid for regardless of how they painted the part.

Gary
....

[Ed S.]

Re: Finishes For GM Bolts and Nuts

Thanks Joe - that is good info. That said, do you know exactly what is the composition of a "plain finish"? If it is not bare metal, what is the coating that is on the fastner that is defined as a plain finish?

[Joe L.]

Re: Finishes For GM Bolts and Nuts

Thanks Joe - that is good info. That said, do you know exactly what is the composition of a "plain finish"? If it is not bare metal, what is the coating that is on the fastner that is defined as a plain finish?

Ed------


That I do not know.

[Ed S.]

Re: Finishes For GM Bolts and Nuts

So this is interesting. I did a google search for "plain finish bolts" - got a lot of good hits. The first one on the list was the Portland Bolt Manufacturing Company. https://www.portlandbolt.com/technic...inished-bolts/. In the company's FAQ section this is what they say about "black bolts".
What is a "Black" Bolt?

The vast majority of bolts that Portland Bolt provides are either hot-dip galvanized or plain steel with no coating. In the construction fasteners industry, we typically refer to the plain steel as “black.” On our quotes and orders at Portland Bolt, we abbreviate that to “blk”. Other companies may refer to bolts with no coating as “bare”, “plain”, or “plain finish.”

The picture below from one of our quotes that says “blk” is short for “black” and is simply referring to a bolt with no coating.
Occasionally, there is some confusion as to what a black bolt means. There are some plating options such as black oxide or black zinc that provide an unmistakably black finish, but in the construction fastener industry, it’s relatively infrequent that those coatings are used. In the event that we are quoting either of those options, it would be made clear on our paperwork.

So, this company says that a plain finish bolt has NO finish - it is bare metal.