If the head bolt threads are rusty, they have zillions of microscopic rust pits in them that absolutely increase friction when torquing the bolt down.
My head bolt threads were rusty and even though I wire brushed them and oiled them, the threads were not smooth because of the pitting. I was very familiar with how they felt when torquing them down.
Wanting to be a perfectionist, I decided to replace all the head bolts because I wasn’t happy with the rust.
The new head bolts torqued down way farther than the old bolts ever did because their threads were smooth as glass. The difference was startling. Without a doubt the head got clamped down much tighter using the new smooth head bolts because they had so much less rotational friction when tightening.
Less thread friction means the torque wrench keeps on turning and clamping the head tighter until eventually… the wrench clicks off.
I remember saying out loud a few times’ “The head bolts never tightened down like this before “.
So yeah - replacing the head bolts if they have rusty threads makes a big difference.
My seat-of-the-pants conclusion was — the Toyota FSM torque spec for head bolts is relevant for NEW head bolts with smooth threads. If old rusty thread head bolts are reused, the torque has to be increased to compensate for the increased friction of the not smooth threads.
How much more increased torque? I can’t say, but I know for certain that torquing old reused head bolts to the FSM spec absolutely doesn’t clamp the head as tight as it was coming from the factory.
For decades, guys have been reusing old head bolts and things have gone pretty well, so it’s a pretty safe bet…. but after torquing down my cylinder head with new bolts….my preference is to replace old rusty head bolts with new.
Reusing them once is probably fine. Maybe twice. I would not want to run them if I knew they had been reused several times.
I’m rebuilding my engine currently and I am fairly certain someone has been in here before me. While I would love to save $150 right now and just reuse my old ones, I have learned my lessons. I will not save $150 now only to spend $1500 later fixing it.