Yeah, okay you win! :biglaugh:
.... But basic physics still applies. Both the bolts and the studs are made from the same material (190,000 psi). Okay, so what's the difference between them. For studs, the nut rotates on the fixed bolt, and the thread friction is both less and stays more even as the nut is tightened. For the bolts, the thread friction increases both because we have a non-ideal thread interface of cast iron and bolt steel, and also because the thread contact length monotonically increases and the bolt descends into the block. As a result, the spec for the bolt to achieve 75% rated strength is 4ft-lbs higher (85 versus 81) and the actual statistical error in achieving the target 75% value is greater because the thread friction uniformity from bolt to bolt is lower than for the studs.
So, conclusion #1 (as you can verify elsewhere on the the net if you don't believe me) is that studs achieve target of 75% of yield strength with lower applied torque and will be statistically more uniform across a given number of them. This is why studs are preferred.
Okay, so what about aluminum. It is common knowledge that the strength of aluminum is lower than steel (Youngs modulus 69 GPa versus 196 GPa for iron), and the thermal expansion coefficient is higher (23e-6 versus 12e-6 /degC).
That means, that aluminum expands twice the amount of iron, and is 3x weaker too. What does the metalurgical engineer do? He backs off the initial cold preload on the bolt because he knows that when hot the expanded aluminum will tighten up the bolt more than for iron, and if he is too tight initially it will hit 100% yield and stretch.
Here's aquote from ARP
"Heat, primarily in aluminum, is another problem area. Because the thermal expansion rate of aluminum is far greater than that of steel it is possible to stretch a fastener beyond yield as the aluminum expands under heat."
Conclusion #2. Aluminum expands more than iron and needs a lower cold preload to prevent exceedin 100% yield when hot.
Note that conclusion #2 is exactly why ARP derates the recommended preload 12% from 85 to 75 ft-lbs for aluminum applications. This derating also applies to the studs, since the physics is the same. For studs, the instructions don't mention anything about aluminum applications, and in my opinion based on the physics that I was taught (at Caltech by the way) that is an error. I would personally derate the torque spec for studs from 81 to 71 ft-lbs for aluminum if I was using them.
Okay, the ARP specs basically assume that we are bolting together two monolithic slabs of metal together. The above specs from ARP consider only the yield strength of the bolts, and nothing else. That of course is far from the case. The head in particular, is full of cavities and holes, is made of soft aluminum, and is easily warped.
I ran a very simple finite element stress analysis (ANSYS Mechanical) of the 7M. It made some gross simplifications to both the block and the head, and uses nominal values for the iron and aluminum since I have no data on the true metalurgical grade that Toyota uses. These results must be taken with a grain of salt, but I saw a very nonuniform gasket loading that got worse as the bolt torque increased. Up to 100 kpsi it looks okay, but strange things happen as the load increases eventually getting very scary as the head/bolt interface pressure actually starts to decrease midway between the bolts as the head get distorted by the bolt pressure. This results seems a little weird to me, so like I said take this model with a grain of salt
My analysis is obviously off, because people are running at ridiculous torques according to the posts here, in some cases even beyond the yield strength of the bolts. Nevertheless, for an engineer like me, I'm not about to use the ARP spec based solely on the yield strength of the bolts. Based on my look at the 7M, I don't recommend the high torques you and others are advocating.
PS
By the way, the auto industry as a whole is moving towards metal MLS head gaskets. One of the big reasons is that a MLS requires *less* preload than a composite. This means the manufacturer can go to a *weaker* and lighter design for the head and block because they get to decrease their bolt torques without compromising performance thereby saving weight, reducing costs and increasing their CAFE !
jdub said:
I've been wanting to see this...it explains where the 75 ft/lbs torque value comes from. This is a spec sheet for the ARP Head Bolts...thank you.
FYI...the thread subject concerns ARP Head Studs
