Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

[Jack O.]

Hi everyone,

I probably missed this but after reading through the litany of posts on hardware finishes I'm still not sure. Is it "generally" true that if a bolt has a zinc phosphate finish it's corresponding lock washer and, if applicable, nut, would also be zinc phosphate? I noticed most, if not all, of the repro bolt sets I get only has the bolt with a zinc phosphate finish. Also, after taking a quick look on line finding grade 5 or better zinc phosphate lock washers and nuts doesn't appear likely.

Thanks,

Jack

[Gary B.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Jack,

I don't know if zinc/zinc or phosphate/phosphate is true in general, but I know of one counter-example. Most of the bumper bolts on '67s are zinc plated, while the nuts on mid to late '67s are all black phosphate.

Gary

[Jack O.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Thanks Gary! I'm working with the trans/shifter and mounts bolts on a '69 at this time.

[Ron G.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

My experience has been 68-72 cars is not uncommon to see some cad or zinc bolts with a black oxide washer. Usually black oxide bolts always have black oxide washers. I cannot say for sure in the reverse.

[John D.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

I have seen black phosphate fillister head scews with cad/zinc lockwashers and also black washers. Rochester Products had ladies that put the fuel injections together and they weren't as fussy as we are.

[Jack O.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Thanks for the input everyone. I've had no luck finding zinc phosphate SAE split washers or nuts grade 5 or better.

[Gary B.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Jack,

I'm confused. What specific GM fasteners are zinc phosphate? What part number, for what year?

Gary

[Jack O.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Gary,

First, I may have the exact name of the finish incorrect but I believe it is either zinc phosphate or black phosphate. I'm working with the hardware on a
69 that mounts the manual trans and shifter. I have some of the original bolts which have the black finish but can't tell the finish of the split washers, flat washers and nuts and was wondering if they would be of the same finish as the bolt it is used with. So a specific example are the 7/16" bolts that mount the trans to the cross member bracket and whether the corresponding split washers would also be "black" or bright finish. I don't have my assembly manual in front of me at the moment but can update later with specific part numbers.

--Jack

[Gary B.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Jack,

Fasteners of that era that were dark gray were typically black phosphate. Sometimes they had an additional oil treatment. Often companies these days that are reproducing nuts, bolts, etc. will use a more modern treatment since black phosphate has minimal durability.

Also, some lock washers during the C2 and C3 era were natural, but heat treated and that gave them a dark gray appearance that is easily mistaken for black phosphate.

Do you know the GM part numbers for the specific bolts and washers you are looking for? They might be listed in the '69 Assembly Instruction Manual.

Gary

[Richard G.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Jack
Here in the Pacific North West, where I live, the only black SAE nuts and washers that I was able to find were Grade 8. They were inexpensive and I could purchase them in any quantity. Vancouver bolt is the name of the company. If you would like some sent to you PM me, or possibly they ship?
Rick

[Gary B.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Rick, Jack,

Some of the black nuts and bolts that the reproduction parts companies sell appear to me to be manganese phosphate, which is more shiny that the original GM black (zinc?) phosphate, and those reproduction items typically lack the oil coating that was common on GM parts.

Three or four people (Grant Wong and Jerry Fuccillo come to mind) who post of the DB have become expert at doing their own plating and finishing, including creating a close approximation to the original black phosphate and oil GM appearance. One can find professional shops that will do the old-fashioned black phosphate and oil treatment, but their minimum charge typically means going that route would only be cost-effective if you have a lot of parts to treat at one time.

Gary

[Jack O.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Yeah, this has become a bit more complicated than I first thought.

So Gary, the specific hardware I'm speaking of is from the '69 Assembly Manual UPC 6 B 4, TRANSMISSION BRACKET & MOUNTING.

I believe the bolts - 9418931, 9418967 and 454933 - are all black phosphate. What I am asking about is the finish of the lock washers (103328 and 103321) and nut 9418931.

Thanks for the pointer Rick.

--Jack

[Gary B.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Jack,

The part number information helps. I can tell you something about the locks washers 1033xx. The spec from the GM drawing for GM 103328 states:

7/16" Washer heavy helical spring lock. Carbon Steel SAE 1060 or Equivalent 38-46 HRC. Finish plain

I believe all GM washers with part numbers 1033xx had the same hardness and finish spec. The Rockwell hardness of 38-46 HRC is quite hard, and that requires a heat treatment process. Based on an NOS example of a lock washer that I have in the 1033xx range, I can tell you that the heat treatment results in a washer that is very close in appearance to black phosphate, i.e., it is quite dark gray, not quite black, and not at all what you'd might imagine at first if you're thinking "plain finish".

Regarding the nut 9418931, I don't know what finish that nut had. If you can search the AIM and see where else that nut was used, perhaps other people will be able to guess as to its finish based on examples on Bowtie cars.

I am constantly on the lookout for GM NOS lock washers in the 1033xx range and I have only found a single example in years of looking on e-bay. Some GM lock washers in the 1033xx series may not have been sold as GM service parts, so one might never find some of them.

AMK Products sells their versions of 103321, 103328 and 9418931, but I suspect theirs are not 100% equivalent to the original GM items. The AMK version of the 103321 lock washer comes either in silver zinc or phosphate, which means AMK is not simply copying the GM spec, since the same GM part number would not be used for two very different finishes. Nut 9418931 is sold in silver zinc by Dr. Rebuild, but in phosphate or zinc by AMK. So who knows what it was.

Gary

[David B.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

Nut 9418931 -- 3/8 x 16 phos. coated GM 4435-M code A, specification. Code A is Phosphate coating on steel: As designated by part number GM 4341-M, code A specifcation requiring phosphate coating plus a subsequent oil type treatment and 72 hour salt spray

[Gary B.]

Re: Does Zinc Phosphate Bolt Use a Zinc Phosphate Lock Washer

David,

Thanks for the interesting engineering specs for the 9418931 nut. I assume the same phosphate and oil treatment was the standard spec for hundreds if not thousands of varieties of GM nuts and bolts. during the C1, C2 and C3 era.

Gary