[Cloud9...68]

With the much-discussed thinning of our cars' 14-17-year-old head gaskets, I'm wondering if anyone has tried simply re-torqueing their cylinder head bolts? I know that I solved 90% of my car's oil leaks by going around and tightening the oil pan bolts, so I don't see why the same wouldn't apply to the head bolts. Comments?

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[flash]

the torque procedure is very odd on these cars - it's not measured by pounds but rather by degrees - it uses a completely different tool - not sure how you would "re-torque" to an accurate setting

further, the edge crush potential of doing that is likely to mess things up worse than leaving it alone

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[Cloud9...68]

That's weird - so a normal torque wrench isn't what you'd use? Very odd...

Many moons ago, I remember it being standard recommended practice to re-torque the cylinder head bolts a few hundered miles after R & R'ing a cylinder head, apparently because the gasket would compress a little under the pressure. I guess this is no longer the case, because I haven't heard of anybody doing it in along time.

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[flash]

nope - a regular torque wrench won't do that job

yup - it is standard practice on any other engine i ever worked on

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[durtkillon]

The difference between the old torquing method and the new is in the bolts. Manufacturers commonly use stretch bolts now for cylinder heads. Stretch bolts are tightened until they deform in their plastic region (think of pulling taffy apart) as opposed to the elastic region (pulling a bungee cord), like the old school bolts. It's more reliable and easier to manufacture a leak proof engine with stretch bolts because the tension is constant and repeatable with new bolts. Manufacturers can also use less of them because they are that good. The thing about tightening the bolt further is that they will quickly reach their fracture point after they have stretched and generally get weaker and less elastic with every turn once they are in the plastic region. For this reason, a torque angle gauge (available for 20 bucks at Sears) is used to return the nut to the proper tension (not torque). Torque varies too much with bolt age, friction thread damage, bolt deformation, etc...

The procedure to tighten our head bolts (attached) is to oil the threads, tighten to 20nm in a specific pattern, tighten 60 degrees in the same pattern, then tighten 90 degrees in the same pattern again. Many people believe these bolts should be replaced each time the head is removed, but the Porsche manual makes no mention of it.

Kurt

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[PorscheDude]

^^Great post.

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[94SilverCab]

Just a comment to Kurt....

That is one very well written post. Straight-forward explanation for why something is done in a particular way, with specific examples and a simile for the layman, and supporting documentation.

That wins my award for post of the week.

Jay

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[PorscheDude]

Or the month, even.

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[ds968]

It has now been about 3,000 miles since my head job. The bolts were tensioned as noted above using the degree turns method. The original bolts were not replaced, they're the same ones . So, should the head bolts be re-tensioned in this case, or no need ?

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[flash]

based on the above, i would say "no"

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[apex]

Just a note that you don't really need the angle gauge - the research I did returned two data points: the cheap angle gauges are not reliable, and some techs who change heads all the time recommended to just eyeball the angle.

So I made my own gauge out of a piece of cardboard with marked angles - worked like a charm - very easy.

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[durtkillon]

Thanks guys. It makes me feel good that I can contribute something to this site. It's been a great source of information for me. I get more from this forum than I could possibly return.

I'm going to get my geek on here, so brace yourselves...

Here is a little more on why stretch bolts are superior to the old style elastic bolts. Since the objective when installing a head is to have the same tension (not torque) for each bolt, engineers look for a way to get repeatable results. They turn to their material science books for the answer. The graph below shows a tensile test that is typical of a ductile material like steel. A tensile test is a very simple test where you take a material, say a bolt, and pull it until if breaks. During the test, you measure the tension (or force) and the elongation (length).

There are three regions of the chart that are of particular interest when selecting the proper bolt for the job. The first is the steep linear region from 0-3 on the chart. This is called the elastic region. As you can see, there is a small amount of elongation relative to the amount of tension applied. This is where the old school bolts were usually torqued to. As you can imagine, the downside to tensioning a bolt in this region is that the slope is very steep. In other words, if you tensioned 8 bolts, they would likely be at very different tensions.

The second key part of the graph is the plastic region (from about 5-17mm). In this region, the metal becomes permanently deformed (like taffy). The other thing that occurs is that the metal becomes stronger, so that even though you stretch it, deform it and the cross section of the bolt becomes slightly thinner, it's maintaining a relatively flat tensile slope. Now, if you tensioned 8 bolts in this region and measured the tensile force, you have a much better chance of repeating your result. This is why engineers pick stretch bolts for the head application. The best way to ensure you are in this region is to understand the bolt's material properties and measure the elongation. This is done by measuring the angle (or threads) and is very accurate. The reason you don't use a torque wrench for this is because there is no additional force needed to continue deforming the bolt, so you would likely continue torquing the bolt right off the graph.

The final region of importance is between 16 and 21mm. In this region, the reduction in cross section is overcoming the increase in strength. In addition, the metal becomes more brittle and evenually...pop! It breaks. Notice that that region is pretty small and trends in the wrong direction. It's a bad place to be.

The bottom line is that the that flat plastic region offers a wide span of reliable, repeatable tension.

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[flash]

ha ha

man that brings back very blurry memories of college - gee, thanks

geek

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[Cloud9...68]

Thanks for the great explanations, which definitively make the case against any benefit to re-torqueing the head bolts.

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