body mount bolt pic

[jiho]

Alright, I'm finally ready to throw in the towel on this and cede defeat, but not because of anything to do with whether GM gave torque specs for dry or lubricated threads. No, it's far more bizarre and complicated than that.

As you can just make out in the snapshot below, my car has Class 12.8 bolts -- not 12.9.

12.8 was an oddball grade that GM started using in these A/G-body cars with the '78 models. 12.8 wasn't even in the official SAE standard, but an appendix to it. Why they used it is arcane, having to do with hardness values where corrosion is an issue. But here's the bottom line: they used it instead of 10.9, but with the same torque spec as they would have used for 10.9. I know this because they used M12 Class 12.8 bolts in the rear control arms until those bolts started cracking, then had a big recall to replace those bolts with 10.9 at the same torque. But clearly they continued using M10 Class 12.8 for the body mounts, as shown below in my '83 Malibu.

So I will use the zinc plated 10.9 bolts I bought, lube the threads and torque them at 40 lb-ft or so, and shut up about this issue. And while I'm at it I also must admit that this is consistent with torque specs in the shop manual being dry unless stated otherwise.

 

[jiho]

As a postscript, the above snapshot shows driver-side position 7, rearmost mount just ahead of the rear bumper, which is easily seen without jacking anything up.

 

[Pontiac455]

We may have to ask our AIRCRAFT TECHNICIAN about this one.

 

[pontiacgp]

Alright, I'm finally ready to throw in the towel on this and cede defeat, but not because of anything to do with whether GM gave torque specs for dry or lubricated threads. No, it's far more bizarre and complicated than that.

As you can just make out in the snapshot below, my car has Class 12.8 bolts -- not 12.9.

12.8 was an oddball grade that GM started using in these A/G-body cars with the '78 models. 12.8 wasn't even in the official SAE standard, but an appendix to it. Why they used it is arcane, having to do with hardness values where corrosion is an issue. But here's the bottom line: they used it instead of 10.9, but with the same torque spec as they would have used for 10.9. I know this because they used M12 Class 12.8 bolts in the rear control arms until those bolts started cracking, then had a big recall to replace those bolts with 10.9 at the same torque. But clearly they continued using M10 Class 12.8 for the body mounts, as shown below in my '83 Malibu.

So I will use the zinc plated 10.9 bolts I bought, lube the threads and torque them at 40 lb-ft or so, and shut up about this issue. And while I'm at it I also must admit that this is consistent with torque specs in the shop manual being dry unless stated otherwise.

that is interesting and I could find much info on a 12.8 bolt....this could be the reason why...

This is from the ASM Handbook:

However, not all of the ISO property classes are used in these specifications for metric steel threaded fasteners. Specification SAE J1199, for example, no longer allows the high-hardness fasteners (ISO bolt classes 12.8 and 12.9), because these two classes are susceptible to delayed brittle fracture in corrosive environments. This change in SAE J1199 is in response to the stress-corrosion cracking of class 12.8 bolts in automobile rear suspensions after just 2 years of service in the Snow Belt of the United States (Ref 1).

...

Ref 1: T.J. Hughel, "Delayed Fracture of Class 12.8 Bolts in Automotive Rear Suspensions," SAE Technical Paper Series 820122, Society of Automotive Engineers, 1982

...

Caution: ISO bolt class 12.9 has also been removed from SAE J1199. Caution is advised when considering the use of class 12.9 bolts and screws because, like the 12.8 class, the 12.9 class is susceptible to stress-corrosion cracking. The capability of the bolt manufacturer, as well as the anticipated in-use environment, should be considered for both the 12.8 and 12.9 classes. High-strength products such as class 12.9 require rigid control of the heat-treating operations and careful monitoring of as-quenched hardness, surface discontinuities, depth of partial decarburization, and freedom from carburization. Some environments may cause stress-corrosion cracking of nonplated as well as electroplated products

 

[t01blaze]

I just went and checked, all the body mount bolts on my 79 Malibu are 9.8...is that bad?

Do I lube the threads and what do I torque them to?

 

[jiho]

Ref 1: T.J. Hughel, "Delayed Fracture of Class 12.8 Bolts in Automotive Rear Suspensions," SAE Technical Paper Series 820122, Society of Automotive Engineers, 1982

That paper was all I could find on 12.8. Cost me $24 at http://www.sae.org. It's from a guy in GM's metal research lab, explaining what went wrong that caused the recall, and the source of my explanation up top.

Caution: ISO bolt class 12.9 has also been removed from SAE J1199.

So that explains why I couldn't find any 12.9 bolts. Good thing, too, the clamping force would have been way too high.

 

[454cutlass]

My best body mount pic. The two behind the rear tires didn't come out nicely.

 

[jiho]

Makes the issue of which bolt to install and how to torque it seem less important, doesn't it?

What continues to gnaw at my little mind is, how could GM have used these 12.8 body mount bolts in this car, a couple of years after the second largest recall in their history (6.4 million cars by their own count, every 1978-81 A-body then on the road), and after publication by one of their own engineers, which led directly to SAE washing their hands of 12.8 entirely?

This car rolled off the line in about June 1983. SAE issued the revised J1199 standard in September 1983. Until then the earlier standard was technically still in force saying 12.8 was ok.

But in the SAE paper (February 1982) it is stated that 506-M, the GM standard for 12.8 bolts, was withdrawn when they did the recall (February 1981): "Concurrently GM specifications covering 12.8 grade fasteners were discontinued."

So my snapshot shows a bolt that was used under a GM standard that had been discontinued well over two years prior?

That's our GM.

 

[jiho]

Gives "planned obsolescence" a whole new twist.

 

[pontiacgp]

I had friends who worked at the GM plant in Oshawa....when they ran out of parts on the line they'd grab anything off the shelf, some people got a posi free of charge. If you worked there and bought a car they's let you follow it down the line and you got the best they had to give you. Those 12.8 bolt were probably old stock that they wanted to get rid of so they used them in the body bushings instead of in the control arms. The 12.8 bolts caused issues in the suspension so the engineers probably ok'd the 12.8 bolts to be used to secure the body in a few locations as that is not as critical as the suspension. But then again who really knows...