The why to using new headbolts, crank bearing bolts-

[wagonteeth]

I've done head work on a 1FZ and have big end bearings to do in a 1HDT. I've always heard the pros and more skilled wrenchers here say it's best to use new head bolts, and the same.goea for the crankshaft caps. I didn't ask why amd just trusted the advice as precautionary.

For those uninitiated, the torque specs for the head bolts instruct to torque to a torque spec and then "rotate bolt 90 degrees".

From the manual image below is the reference to plastic region tightening bolts-

I didn't know about plastic region tightening bolts, so I had Grok explain...

"The plastic region tightening of bolts refers to a method of tightening bolts beyond their elastic limit into the plastic deformation region of the material. In this process, the bolt is subjected to a controlled torque or tension that causes it to permanently deform, ensuring a high clamping force and a secure joint."

So it's not just good advice, but a specific engineering element for the strength of these critical connections.

 

[mudgudgeon]

They can potentially be re-used IF they measure up within the specs in the manual.

Best practice is replace them regardless.

I've re-used them in the past and have had bolts yield before reaching specified torque + 90⁰.
The time and hassle of waiting for replacements vs finishing a project makes it not worth re-using them.

To have one fail during service after an engine rebuild, and having to go back in and repair a failure, and subsequent damage would cost exponentially more than new bolts.

If you're on a really tight budget, it's tempting to re-use.
If you're on a really tight budget, you can afford to cut corners. (and probably can't afford a 30yr old landcruiser).