The overall design of the exhaust manifold is bad. First, the thermal expansion of the aluminum head and the cast iron manifold are quite different and no compensation was designed into the mounting or the manifold. To make matters worse (I believe this to be the real problem), the engine was offset to the passenger’s side, so there’s no room to have a protruding manifold with a few turns/bends in its design (like older style manifolds on cast iron blocks) to allow for the mismatched expansion. To make things fit, they came up with a design that looks like a log, so all expansion & contraction puts stress on the outside bolts/studs.

The torque on the manifold bolts is low and the gasket is special – I’m assuming this was at least in part a “fix” to the poor design. The gasket itself is very slippery and the manifold bolt torque is less than 20 foot pounds (it’s 18 I think), so this would give the manifold some mobility to minimize the bolt stress, but it’s still not enough to stop sheering the rear manifold bolts. The low torque and aluminum heads should make the bolt shaft extraction a lot easier. From my research, I’ve found that most of the people that had the bolts sheer with some of the shaft protruding, were able to extract them using vise grips. Mine was sheered flush and I drilled a hole in the center and with good penetrating oil and an easy out, I was able to coax it out without too much effort. If that had failed, I was set up to use a welding procedure to extract the shaft so that I wouldn’t screw up the threaded hole in the head.

 

Would drilling the bolt holes oversize a bit help? Or maybe just elongate the end holes by hand w/a rat-tail file?

 

Off-hand, I don't think that would have a great deal of effect. Looking at the one end-bolt that broke on my manifold, I see that there's a lot of bolt sticking up beyond the engine head. My guess is that the thermal expansion & contraction is able to rock the protruding bolt section back and forth and after so many back & forths, the bolt is weakened enough to break - this would make it a fatigue break rather than being sheared. Th is could be why you have some bolts that are broken at the block while others leave more of the shaft exposed. On re-assembly, I decided to consider a potential "next time" & used anti-seize compound on the bolts to make the extraction easier.

 

If thermal expansion/contraction of the manifold is causing the bolt to flex until it ultimately breaks, I would think the relieving the holes would take the side pressure off the bolts and stop them flexing.
That's my opinion. YMMV.

Talk slow, think fast.

 

In the pictures I have seen, the bolts look like they failed in tension, not shear. If they were shearing, they would all be broken at the head surface.

 

You might be right, my assumptions are based on the following:

• The bolt shafts are currently a loose fit through the manifold
• The bolt heads are about an inch or more above the cylinder head
• The bolt heads are snug to tight against the manifold, so they would tend to move with the manifold & making the manifold holes larger wouldn't have much of an effect. The back & forth motion wouldn't be that large, but it would be constant. With a large motion, the bolts would fail a lot quicker.

Keep in mind that this is all conjecture on my part based on what I saw on my old truck ((now my son's).

 

Northgator8, I've only seen the one bolt that broke on my Dak, but it didn't have the typical signs of a tension break or a shear break. This is what got me thinking about a fatigue break.