LMWatBullRun wrote:
You are going to get stochastic variation on installed bolt torques. Good manufacturing practice is to select procedures that guarantee minimum clamping force on all bolts without exceeding maximum clamping force on any bolt. If VMM had used hardened and ground washers under the bolts, for example, they would have eliminated the galling and friction between the head and the bolt head that is one source of torque variation. They also would have reduced the deformation of the head bolt recesses that Geordi and I and others have seen. Some engines are undoubtedly going to get 16 good head bolt installs, perhaps the majority of them. But others won't, as has repeatedly been demonstrated. It is a crap shoot, due to the unpredictable nature of the bolts selected.
the really nice thing about the ARP studs is that you get predictable, reliable clamping force EVERY TIME from EVERY STUD. I am convinced that this modification is one of the two most important mods one can make to ensure proper engine function, right up there with dealing with the soot problem.
Arriving at an acceptable torque factor that gives you the minimum clamping required, yet does not exceed the maximum structural strength of the construct is called "good design". I would suggest that the space between those two numbers should be fairly large... EXCEPT in the case of the CRD where the head will yield (failure) not far above the measured torque values of the factory bolts. This is not a good design. You always want a normal bolt to remain in the elastic region, so that it does not exceed its own tolerances. TTY bolts by nature do not do this, IF they are properly sized to the job. I am also uncertain about that, because of experiences on this forum and in this thread.
A better design could have been a reinforced cylinder head that didn't crush so readily, where torque values (needed at 130lb-ft) did not cause failure until 150+ lb-ft. As it sits, the deformation could happen because of variations in the tolerances of the torque or angle wrenches used by the factory, because even they would be expecting a larger working range between elastic / plastic and failure. Perhaps the component of the TTY structure the design was expecting to yield was the head, and not the bolt? Could it be that badly designed?
The point and debate is somewhat irrelevant however. Practical experience has shown that these TTY bolts should be replaced for the ARP studs, somewhere around the 100k mile belt change. The vast majority of coolant leaks have been right around 130k miles, so there are the numbers. The result will be predictable clamping, and even the lunatic rigging his CRD with compound turbos pushing 30psi has not had one whiff of a problem by having the ARP studs installed. For the general user, this is probably a bit of overkill, however:
When engineering, the proper amount is over-engineered. That is called "safety factor".
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