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Discussion Starter · #1 ·
I have been putting off a head gasket repair for better weather. Looking into the radiator for bubbles and tailpipe and in the engine compartment, it seems that it is leaking to the outside rather than into a cylinder.
Now, I am not sure of the mode of failure. With 170,000 miles, I am wondering if the preload on the bolts is suspect and, if I re-torque them, would it solve the problem. The mode of failure may not mean a bad gasket but loss of preload.
I am really not sure of how a gasket fails. Is it the constant temperature cycles that breaks down the mechanical strength of the material?

As an analogy, tapered roller bearing lose preload after the ends of the rollers burnish in. An early practice was to measure the torque required to turn a race. After a few hundred revolutions, a noticeable change in torque could be detected. In fact, old bearings are typically preloaded to 50% of their original torques. Now, end play is measured. More accurate but also more time consuming.

Anyway, I am thinking of initially retorquing the heads but this leads to the question "What do I torque to?". Since the initial torque also accounts for a squeezing and seating of a new gasket.

Maybe this notion is incorrect.
 

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At 170,000 miles on what I'm assuming is the original factory gasket, I doubt that a loss of preload is the cause of the failure. Those bolts will be siezed up real good and I don't see them backing out on their own. I also don't see a material failure because if that's the case, it should have went a long time ago. Its's the gasket. Did you overheat the engine at all? That's what usually takes out head gaskets (that and running too much boost on a forced induction motor).
 

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Discussion Starter · #3 ·
Dealing with large mechanical equipment that can see great force applied but small temperature ranges, I have encountered bolts that have been stretched. Writing this and not having my ARP catalog in front of me, I recall that head bolts are typically loaded to 80% of their yield strength. Which is why everytime one changes a head, new bolts are recommended. This is why one should never use a standard tap to clean threads but a thread chaser-to prevent the possibility of re-cutting the threads.
But the 70-80 ft-lbs of torque on a new bolt and gasket is used in seating the gasket. So, I'm using some Kentucky Windage here and saying that I will torque the bolts to see what I have then go from there.
I am also thinking about the relationship of the newer aluminum heads v the cast iron heads. Since a lockwasher is never used in this application and the aluminum is softer, there may be a difference in holding power.

And, yes, the daughter was driving the car during the summer around town and I wasn't diligent enough to catch a low fluid level. It never stuck the temp needle but I assume it may have but nobody is confessing. Besides, the rocker arm gaskets need changing andother stuff.

Funny, but this is one raggedy old car that nobody is saying get rid of it. It has been reliable and always started (well maybe almost always-there was a time, 80,000 miles ago when it needed a fuel pump).
 
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