'66 upper A-arm to frame bolts; GM 3888248

[Gary B.]

The bolts that held the upper A-arms to the frame on my '66, which I believe to be original to the car, should be GM # 3888248. My bolts are 7/16-20 x 2.125" long. The bolts are rusty and there is no indication remaining of the original finish, but based on the rust, I suspect they were black phosphate, rather than zinc or cad. The '66 TIM&JG is silent on the finish. Can anyone confirm they were black phosphate?

GM 388248 was available in service. Does anyone know the finish of the service part?

Thanks,

Gary

[John D.]

Re: '66 upper A-arm to frame bolts; GM 3888248

gary,

The drawing shows "Bolt special head 7/16-20 x 2 1/4"
Material 300-M steel, no finish specified- phosphate or otherwise.

[Gary B.]

Re: '66 upper A-arm to frame bolts; GM 3888248

John,

If no finish is specified on the GM drawing, does that mean that any finish the vendor chose would have been acceptable?

Gary

[Gene M.]

Re: '66 upper A-arm to frame bolts; GM 3888248

I think you will find from disassembly of originals the finish was black. I'm not sure if it was phosphate or a black oxide. But I fairly sure it is not black zinc. Most all high grade bolts were not zinc as it causes hydrogen brittleness with out heat treatment.

[John D.]

Re: '66 upper A-arm to frame bolts; GM 3888248

gene,

The drawing does say "decarburization classification C or better". I don't know if that process imparts a black finish or not.
There may well have been a finish on these bolts... it's just not called out on this drawing.

john

[Gary B.]

'66 upper A-arm to frame bolts; decarburization spec

Gene,

I think you are correct on the black finish, but I've talked with one other person who is restoring a '66 and he plans on using zinc plated hardware.

Are there any metallurgists out there who would know whether the decarburization spec would apply to a bolt that was zinc or cad plated?

Gary

[John D.]

Re: '66 upper A-arm to frame bolts; GM 3888248

I consulted with a friend who is a Fastener Engineer here's what he had to say:

The decarburization note is not a finish but just a specification to have them verify they did not cause excessive carbon loss during the normal heat treatment process.

I would expect the bolt would appear black and oily. A plain finished bolt like this would end up getting phosphate and oil to keep it from rusting prior to installation. Once it hit the weather, this finish would end up rusting pretty fast. The phosphate is just a wash in zinc phosphate to create a porous matrix of crystals on the surface. It is then dipped in the oil which fills in the porous holes and creates the coating. Without the phosphate, the oil would quickly drain off the bolt and into the box it came in. This finish was very common on our bolts. It was pretty much the standard.

[Gary B.]

Calling Joe Lucia

Joe,

Do you have any NOS 3888248 in your collection? Are they black phosphate?

Gary

[James E.]

Re: Calling Joe Lucia

Gary:
I have some NOS upper control arm bolts,PT# 381909. Bought back in the 80's,they are listed for 68-76,they are black.Hope this might help.

[Gene M.]

Re: '66 upper A-arm to frame bolts; GM 3888248

The replacement upper A arm shim bolts are 7/16 course thread for c3's. The c2 originals were 7/16 fine thread. They (course thread) will fit and function perfectly but may not flight judge well if you have a knowledgeable c2 chassis judge.

[Gary B.]

Anyone remember what Joe Tripoli lists in his book...

for the finish of these bolts?

Gary

[Dick W.]

Re: '66 upper A-arm to frame bolts; GM 3888248

All the ones that I removed appeared to have the black finish in the protected areas.

I think that John Daly answered the question. With his knowledge and being a GM employee, I trust his answers

[John H.]

Re: '66 upper A-arm to frame bolts; GM 3888248

As far as I know, they're standard GM phosphate-and-oil; have never seen them with any other finish.

[John D.]

Re: Calling Joe Lucia

Gary:
I have some NOS upper control arm bolts,PT# 381909. Bought back in the 80's,they are listed for 68-76,they are black.Hope this might help.

James

The 382909 bolt drawing shows a 7/16-14 thread with no finish specified. It must have the same standard black phosphate finish as the 3888248 with the 7/16-20 thread...

jd

[Gary B.]

GM 3888248 vs 382909

John,

Sounds like another nail in the zinc/cad finish coffin.

Does the GM drawing for 382909 also specify the same decarburization class C spec?

Gary