Can head bolts go "bad"?

[creia]

Gentlemen,
My previous thread on my broken head bolt woes got me to thinking....
Do cylinder head bolts have an expected/useful service lifespan? That is to say, over time (and being constantly under torque) should they be replaced with new ones when taking an engine apart and rebuilding? On these old engines (30-45 years old) that my son and I work on, I do not believe that on some of the engines the head bolts have EVER been removed! We always inspect them (they always "look" good), clean up the threads, and re-install the original. I beginning to re-think this and wonder if that is prudent. Does the original "temper, "tensile strength" (or whatever the proper term is) of the head bolt degrade/decline over time?
As always guys, I look forward to some education here.
Regards,
Michael

[bgsengine]

Generally, they will hold up well - it takes some very high temperatures to really affect the temper and strength of the bolts - There are a few caveats:

First, if a bolt is severely corroded (combustion gas, rust, etc.) , stretched (over-torqued) or had been "tight" (much like your broken bolt - but experienced techs have turned removing those jammed up bolts into a Zen practice - Become one with the bolt!) they will be replaced, but that is relatively rare.

Secondly, on *some* engines, the bolts are a "Torque to Yield" or "Torque Angle" - notably some Kohler Command Pro OHV engines, some powersports engines, etc- These bolts are *one time use only* because , as the name implies, you torque the bolts down to their "yield point" where the bolt is stretched beyond any hope of it returning to it's original length upon removal - These are *ALWAYS* replaced, and O.E.M gasket kits typically *include* new head bolts. (and typically the head gasket is MLS - Multi Layer Steel)

Other than that, most head bolts are not so severely torqued that they cannot be re-used, Even after 50 years. However, on old engines, a close inspection of the bolts is always a good idea after cleaning and look for any brittleness (Bright black and shiny, even after cleaning to bare metal) , elongation (typically at the point where threads end and shank begins) and of course, corrosion and damaged threads (damaged threads will affect your ability to achieve proper torque)

[jdm]

And ....

Always clean the bolt thread on a wire brush. If possible run a very good quality (sharp and clean) tap down the thread.
Turn forward 1-2 times, back 1/2 (etc) This helps clear the crap into the channel on the tap. Be very careful not to cross thread it.

Doing this mean that when you torque down, you're torquing against the head gasket compression and not the dirty thread.