Broken exhaust manifold studs ftl

[midnight]

So I have started hearing a ticking sound from the passenger side of the motor when started cold. It goes away after a minute of driving. So I finally get brave enough to get out in the 200' heat to take a look. The back two bolts are broken off at the head. I am afraid to find out how much this is going to cost me. Is this common and what the hell causes it?

[Trail X]

If they are torqued down too far, they can pop when the manifold expands.

It's possible that your manifold was taken off sometime and not torqued properly. How much of the screw shank is visible? If it broke close to the head, it may not be impossible to remove. They are stubborn to remove - even with the head on.

[midnight]

I have owned it from about 50k miles. So it had to have been done before then. I did some reading saying that they used grade 5 bolts on them. Which means I will be replacing with grade 8 when I fix this.

Way to save $3 chevy.

[Trail X]

Careful - Grade 8 are stronger, but also more brittle.

If GM specifies grade 5, it's probably because they "stretch" better, which is needed in high-heat variation areas like headers. Your bolts have to expand and contract properly with the header flange. This is one place where I would probably buy genuine GM parts or other bolts specified for headers.

[midnight]

Good point James.

[glfredrick]

Careful - Grade 8 are stronger, but also more brittle.

If GM specifies grade 5, it's probably because they "stretch" better, which is needed in high-heat variation areas like headers. Your bolts have to expand and contract properly with the header flange. This is one place where I would probably buy genuine GM parts or other bolts specified for headers.

I'm not sure that your explanation is precisely true. The level at which a grade 8 bolt shatters is much higher than the grade 5 under any conditions. The tensile strength of a grade 5 bolt is typically 120,000 pounds per square inch, while grade 8 bolts are typically spec'd at 150,000 pounds per square inch. Both are "ductile" meaning that they will deform under pressure and not just snap (brittle). That would not be useful for bolts, especially ones designed to hold superior load limits under either ductile or shear strength conditions. There are some bolts that are not ductile, but they are not commonly found in automotive use.

The information above was gleaned from Mark’s Standard Handbook for Mechanical Engineers.

Grade 8 bolt capability in yield (stretch) = 130,000 lbs / in2 x .03349 in2 = 4354 lbs minimum
Grade 8 bolt capability in tension (failure) = 150,000 lbs / in2 x .03349 in2 = 5024 lbs minimum
Grade 5 bolt capability in yield (stretch) = 92,000 lbs / in2 x .03349 in2 = 3081 lbs minimum
Grade 5 bolt capability in tension (failure) = 120,000 lbs / in2 x .03349 in2 = 4019 lbs minimum

The grade 5 bolt has already broken before the grade 8 bolt even begins to deform.

Additionally, Grade 5’s typically have a core Rockwell hardness of C25-C34 whereas a grade 8 typically has a core Rockwell hardness of C33-C39. Grade 8 are tougher and more impervious to outside forces, such as the shear of a winch plate or tow point bracket.

There are likely other factors at stake for the exhaust manifold bolts, however, that make Jame's suggestion of using factory parts valid. Bolt shoulders, intended stretch, etc., all factor in, but that being said, I've run a lot of grade 8 hardware on exhaust systems including headers over the years.

[Trail X]

You're talking stress only... you need to bring strain into the discussion.

Below is a stress/strain graph for a few grades of bolts.

The grade 8, as you can see is stronger (higher max Y), but the fracture point is almost half of the elongation of the grade 5 (x axis - less X axis means more brittle). This may not matter over a few heat cycles of the exhaust, but every time you're stretching the bolt, you're fatiguing the bolt. So over more displacement cycles, you're fatiguing the unstretchy bolt more than the stretchier bolt.



This is the case in this situation because the flange is expanding a certain displacement each time it heats up. If you're just imparting force and not displacement on the bolt head, then it makes sense to go with a stronger bolt (grade 8).

[Philberto]

Why do I have a feeling that James had that graph lying around somewhere?

[fishsticks]

Why do I have a feeling that James had that graph lying around somewhere?

[midnight]

he carries it around like a gun. ready to draw at anytime.

[HARDTRAILZ]

There is a very specific tightening sequence to the exhaust manifold. I was told by a couple mechanics that you need to torque them down 5-7 times and it is weird because they torque to the 15 lbs or whtever it is then by the time you follow the sequence and go back to check you have to re-torque. I definitely would use factory parts and follow the proper instructions.

FYI--I discovered this since my O2 sensor was stripped and was worried about breaking manifold bolts to figure a way to get it out.

[glfredrick]

I agree with the stresses that James posted with one caveat... The grade 5 bolt will have already broken -- even at peak stress capability -- before the Grade 8 even starts to work.