Quoting broxma:
I suppose we would have to first clear up what we mean by stronger. Clamping force? Corky's assertion was that the stud would have more clamping force. His explanation, and his degrees and 50 years of experience, are good enough for me to count his assertion as one of some authority. To put forth a completely unscientific premise, why when looking for a "stronger" hold, clamp if you will, for the factory head studs, do we switch to studs? The opposing force is not on a single plane(back surface of the bolt head), but spread on the entire surface of the threads of the nut and the back surface of the nut(bolt head). Never mind that the material may be different as I caveated my first claim with the condition, of same size and material. In addition, when applying torque to the bolt, you have to move the entire bolt, meaning all thread surfaces will be in direct contact and eating away at the opposite thread in the block, nut dilation I believe is what this is. In the case of the bolted manifold, once the bolt head makes contact with the washer(I am assuming a washer was used), any more torque is simply eating that much further at the threads. The iron of the manifold isn't going to give way. The gasket will to a degree, but once it is at its maximum compression, it's all eaten threads. Ideally, the bolt should break before the threads strip out and not because it bottomed out. The stud eliminates all of this and puts all the nut dilation at on the outside at the nut/stud joint, not in the head. My biggest fear about bolting the manifold would be the constant reduction in head material caused by the installation and removal of the bolts. Contrary to what anyone says, this torque and forget it idea is not going to happen. After even just a few cycles with the bolt, the head is going to be in much worse shape. I don't think anyone would argue against that claim.
But wouldn't the threads of the stud also have a higher effective thread engagement? Aren't the threads at the nut calculated into that total figure? I read an equation which stated a bolt has an effective thread engagement of the thread area plus 1/3 the bolt head height. A stud has the effective engagement of the thread area plus 1/2 the nut height. Wouldn't that include the threads of the nut? Or is the additional 1/6 due to the thread of the nut inherently? If it were, wouldn't it be variable upon the nut height instead?
Honestly, I am not able to professionally argue my position. I do have a limited experience with such things but not enough to make a qualified claim. I do trust Corky and his claim though, as it makes logical sense to me and his diagram and explanation were enough to satisfy my question. The overwhelming opinion on the matter from ARP and other bolt/stud manufacturers is that a greater clamping force (Strength as I mean it) is possible with a stud/nut versus a bolt. At the very least they claim a more accurate torque.
That's about all I have on the matter.
/brox
I think your second to last sentence summarized the key benefit besides those mentioned earlier above; accurate torque. Just because a stud/nut and bolt have a different bond between them doesn't mean the forces are any different. The only force in either case would be directly away from the head. I also don't see why thread purchase is multiplied in the case of a stud and nut combo over a bolt when the forces on the threads on either side of the exhaust flange are actually in the opposite direction.
Yes, maybe it is ideal for a bolt to shear off before tearing threads out in reference to your comment, but clearly, at least in the case of our engines, that is not so. Otherwise there wouldn't be such a necessity to heli-coil for so many people. When you're talking about running a hardened steel bolt into an aluminum threaded hole, there's simply only one way to in crease your thread purchase; increasing thread surface area (ie. the total area of the threads acting against the clamping force, or the area of aluminum to bolt material). This would either be done by using a wider bolt, or a longer bolt, both of which would expose more thread area.
One of the first few posts
here says exactly what I'm trying to say, only in better words. It's an even worse problem in this case because cast iron is stronger than a 4g63 head's aluminum.
"The real strength of studs comes from NOT wearing on the threads they are screwed into. Every time you screw any grade of bolt into cast iron threads, you will lose some of the threads. If you can LocTite a stud in place, you won't be stressing the cast iron threads every time you disassemble the engine."
The best option in this case truly is to helicoil from the start. This allows you to use the factory design fastener in terms of depth and width, but gives you more thread purchase area as the area between steel and aluminum has increased. I can't find any references, but many times helicoils are used from the get-go even before a thread has had a chance to strip.
. I may not know what I'm talking about, but I'm thinking the main reason Mitsu puts studs on the exhaust manifold is for serviceability. The less you touch the threads into the aluminum, the better.
