Recently I replaced the head gasket on a 1999 Mitsubishi Mirage with the 1.5L SOHC.  While torquing down the head bolts according the the tighten, loosen, tighten, turn another 180 degree procedure two of the bolts got very loose while turning the additional 180 degrees.  I was a little dumb struck and couldn’t explain it.  I decided to carry on anyways.

When I started the car for the first time it was spewing out white smoke (burned antifreeze).  It was kind of cool, the smoke clung to the ground like a creepy low fog…

I was bummed.  Here I had spent several days changing the head gasket and it wasn’t sealing right.  I decided perhaps I just hadn’t tightened the bolts properly because I had used new bolts, we had the head machined flat, and the block was steel so I wasn’t too worried about that warping.

I got online and did some research hoping for a simpler alternate torquing procedure.  What I found instead is an explanation of “Torque To Yield” head bolts.

If you do much work on cars you may have noticed that newer head bolts (starting around the mid 80′s I think?)  are much smaller.  These are “TTY” bolts.  These are bolts that when properly torqued are tightened past their “yeild point“.  When a bolt is tightened beyond it’s yeild point it is stretched in a way that it WILL NOT return to it’s previous shape.  Most materials have an elastic region where as you stretch it it will always return to it’s normal shape and then a yeild point (which the the absolute most you can stretch it and it still retain it’s original shape), followed by a “plastic region“.  In this plastic region the metal still stretches but it will NEVER return to it’s original dimensions.

What this means is that unlike older head bolts the head bolt can only be tightened ONCE.  It will never return to it’s original shape.

So why do cars use these use once head bolts?  The answer is simple, it provides more even and constant head to block pressure.  You see, when a bolt is stretched in it’s elastic range, the pressure the bolt maintains on the surface goes up VERY fast as you tighten it more (notice the STEEP incline of the red line between the beginning the the point marked with the 2).  However, when you get into the plastic region the pressure on the surface the bolt exhibits changes much less as the bolt is stretched while the engine heats and cools, etc (from mark 2 to mark 3)…

The reason my head bolts failed, is that when I took them to 14 ft lbs I over tightened them a little (my torque wrench is not well calibrated), and when I put the bolts in the plastic region by tightening them another 180 degrees I passed the “Ultimate Strength” (see image below, it’s where the red line peaks). When I passed that point, the bolt began to hold LESS as it stretched instead of more, and the torque of the bolt declined until at the end of the red line it broke.

Back to the car…  I wasn’t sure last night exactly why the bolts got loose after tightening but it felt just like a stripped bolt.  I figured would try to tighten them, and if it broke off, I would use an easy out, and if it is stripped I’ll have to take it apart to repair the threads.  I tightened the bolt another 360 degrees (approx) and SNAP the bolt broke.

It was an easy job with an easy out, and a McGuiver style electro-magnet to pull out the broken part of the stud. (Usually when a bolt breaks, it spins right out of it’s hole because the pressure is removed, as was the case here.).  Tomorrow I’ll order a new set of head bolts, and use my friend’s borrowed craftsman beam torque wrench (beam torque wrenches are generally more accurate the click style torque wrenches) to install the new head bolts and hopefully the car will be fixed (fingers crossed and prayers said).

So the basics of all of this…  If the head bolts never surpasses their yield point you should be able to re-use them as long as the threads stay in good condition. However, if the bolt ever surpasses it’s yield point, it needs to be discarded and replaced.  So, big giant head bolts, may be safe to re-use (unless the engine over heated in which case it is more likely the bolt was stretched beyond it’s yield point), smaller diameter head bolts (especially ones with a torque procedure that has you tighten, loosen, tighten, turn N degrees) should NEVER be re-used.

(Guide to reading graph) In Engineering terms the “Strain” on a material is the distance it is stretched, while the “Stress” can in very simple terms be called the force the bolt exerts on the materials it is holding.  So as you can see, stress shoots upwards EXTREMELY fast while in the elastic region (usually linear), and then enters the plastic region where as the bolt stretches the force stays much more constant, but once the ultimate strength is surpassed the stress and force decline until the bolt breaks.

Tags: Cars, Engineering, Torque Torque To Yield