geordi wrote:
The problem is that there isn't a "spec value" as such because these are TTY bolts and the factory process says "turn x degrees after reaching 60lbs torque" or something like that, and then turn the bolts another x degrees - So what is the final torque of the TTY bolt? Impossible to know.
What we are seeing (and the point of this research thread) is that the #3 bolt seems to be affected by the heat from the cylinders and the exhaust, and loosens. Whether this is from stretch or rotation - I think it is from stretch - is somewhat irrelevant. The studs won't do either when they are properly installed.
On a more general note to this topic, what we see are widely varying release torques, ranging from less than 80 FPT to over 200 FPT.
That indicates that the head/headbolt system is not properly designed.
More interestingly, on my CRD, the amount of take-up rotation required to reach 100 FPT with the ARP studs varied significantly, indicating that the head was slightly warped and that the ARP studs pulled the head back into alignment.
In point of fact, the factory head bolts do
not yield, as torquing them to the point that they yield causes head crushing. Torquing the ARP studs to the compression value required to make the factory "TTY" headbolts actually yield causes yielding of the glow plug and other head penetrations. See the ARP Stud thread for the details.
The point is that the head bolts are poorly engineered for this application, and the ARP studs are a superior solution.
Keep in mind that the beancounters don't care what happens after the warranty is out, which is why they took out the hardened washer under the head bolt and used a 7 mm shorter bare bolt. As long as the engine runs 100k they probably consider it a win.
Factors that affect the release torque include but are not limited to (in no particular order):
-smoothness and flatness of the underside of the head bolt head;
-Galling of the head;
-Compressive force normal to the head/head bolt bearing surface;
-smoothness of the block threads;
-Smoothness and uniformity of the bolt threads;
-Linearity of the bolt.
The ARP head stud addresses each of these issues and is distinctly superior to factory head bolts.
I was convinced of this before I did the replacement, and all of the evidence I have read since reinforces that
opinion. Can you make the engine run without them? Sure! Will the engine last longer and run better with them?
Yes.
The question you have to ask yourself is:
Who benefits from a cheap solution ("TTY" head bolts)that requires regular out of warranty repairs?