1994-2002 [Subscribe to Daily Digest] |
So you did not have to deal with the studs behind the PS pump and turbo feed pipe? Things are almost impossible to wrench in there. Which studs were they? I would expect the ones nearer the ends.
Were you able to get a T-wrench on these? (18,5 ft-lb) Many 1/2" drive t-wrenchs are out of range at that point.
That is a good point, about the studs not been tight. So never tighten a stud hard into the block. You want to be able to remove them easily. So when removing manifold studs, it is not unusual for the nut and stud to be locked up from rust and the heat. So the stud and nut will come out together. They can often be reused after the nut and stud are cleaned up. But seeing how the studs often fail in this application, perhaps in all turbo applications, it might be wise to use new ones.
When installing head studs, they go into the block 'finger tight'. The threads in the block are course, especially for an alloy block, and the threads at the nut end are a finer pitch. So when tightened, the nut will move on the stud and the stud will not turn in the block. ARP head studs are now broached for many applications (all?) so an hex key can be used to backout or lightly spin in the studs. (In many applications, for removal of the heads with the engine in place, head bolts are the norm as studs dictate head travel when been removed that will hit master cylinders etc. For the Saab heads, the head cannot be installed or removed over studs as there is interference with the timing chain gear. But with hex head broached studs, these could be easily used as they are easly to install and remove with the head in place... not that I have had to do such -knock on wood-.)
So why do these exhaut studs break? They are a hard and tough heat resistant steel. The manifold expands and contracts a lot with every engine heat cycle. The block temperature is controled by the thermostat. But the manifold can get cherry red or even a yellow heat. The manifold moves and slips on the manifold gasket. The manifold heat can also vary with each run on boost too. This expansion puts cyclic lateral loads on the studs. The standoff sleeves on the studs may help to reduce those lateral loads on the studs by allowing for longer studs as well as letting the sleeve absorb some of the movement. So as long as things do move, it would probably make sense to not have the studs tigher than neccessary to contain the exhaust gasses. As the manifold moves so much and so frequently, the use of an aluminum head might actually reduce the differential expansion effects as the aluminum will grow to follow the manifold in a manner that a cast iron head cannot. But then we have the problem of the head moving relative to the block and the wear and tear that puts on the head gasket. With the new 9-3 engine, the use of an aluminum head and aluminum block will eliminate perhaps all of that head to block problem.
The studs are thus sujected to a cyclic bending load, moreso at the ends of the manifold. The thread grooves in the studs create a stress riser and cracks will form at the root of these theads at the block interface. I guess that this would have to be regarded as fatique failure.
Manifolds are also highly stressed from thermal expansion, and they can crack. This is why there is a concern with thermal wrap on an exhaust manifold. The manifold then gets much hotter and the added stress combined with less strength at those temperatures can be more than the materal (and studs) can take. The heat shield added in ?1997? do probably increase stresses on the manifold and studs.
Note that many exhaust manifolds have splits or saw cuts to eliminate having one rigid length of cast iron from one end to the other. In a V8, the manfolds do not have to be longer than one on an inline 4 cylinder engine (I4). But on an inline six (I6), things are really getting long, and expansion considerations become all the more of a concern. I don't recall, but the next time I see my brothers XKE engine, I will have to note how that was handled. It is a nice piece, smooth and covered with black vitrious enamel.
posted by 65.68.1...
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
