Quick question

[Jcb]

Sorry for sounding like a half wit, but i've never really got why you have to change the head bolts when you remove the head from the engine block.

I'm sure it's a simple anwser, I just couldn't come up with the anwser by just thinking about it. And i'm too lazy too google :P

[meritlover]

well lucky for you, im not too lazy to type a reply.

to get the correct tension on the bolts you have to stretch them, which alters the molecular structure of the steel. when removed they dont return 100% to the original shape, so further tightening/reusing would yeild them causing them to:
1. not reach the correct torque (and therfore not clamping the gasket with a sufficient force)

2. stretching to the point where they bottom out in the holes (giving you a false torque)

3. shearing in the head.

they are so cheap, they are as well being replaced.

[Jcb]

oh ok, thanks. Makes perfect sense.are the threads in the engine block "progressive" or something (i.e. starting with a shallow turn which gets deeper?) to achieve the stretching you mentioned? Oh I know what i mean anyway....

[meritlover]

no its a uniform thread, stretch is dependent on the pitch of the thread. ie a 1.2mm thread will stretch 1.2mm per revolution (after the gasket has crushed) this is why the setting is usually an initial torque (to crush the gasket) followed by angular stages to get acurate turns on the bolt to ensure the correct stretch is achieved.

eg 4 passes of 90deg ensures an even load across the head on a 1.2mm thread to give (1 turn) and a stretch of 1.2mm.
depending on the tensile strength of the bolt, this will determine the correct clamping load. for example if the bolts were made of cheese, a stretch of 1.2mm isnt as tight as a stretching steel....