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In some cars like some Peugeot & Citroen models, there is a specific stage during the process of tightening the head bolts. For example, in Haynes manual of Peugeot 405 it is stated that head bolts should be tightened as per the following stages:

Stage 1 - Tighten to 60 Newton Meters (nm)

Stage 2 - Fully Slacken all the bolts in sequence

Stage 3 - Re-tighten to 20 nm

Stage 4 - Angle tighten all the bolts a further 300 degrees

I have no problem understanding the stages 1, 3 & 4 but there is something about stage 2 that doesn't make sense to me.

Since it refers to angular tightening, it is clear that the head bolts are Torque-to-Yield bolts and as you know these bolts should not be re-used after being torqued down once. Torquing the bolts in stage 1 and slackening all of them in stage 2 and re-tightening them in stage 3 means re-using TY bolts and that doesn't make sense. In addition to the bolts, the head gasket itself should not be re-used after being put under pressure once. Re-tightening the bolts, slackening & re-tightening them means the head gasket is compressed, decompressed and re-compressed which again defies logic.

So then, what is the main point behind the stage 2 of slackening the bolts in the middle of the process?

Here is a screenshot of the manual:

enter image description here

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    Right after what you are pointing out, it states "see text" ... what does the text say? I suspect there's more to what is going on in the reassembly part of the manual. My hunch is, the idea is to put a preload on the head gasket so as to crush it to size, then release it so when you put the 20 Nm back on, you a more true indicator of how much torque you are actually applying to the head bolt. As I said, just a hunch. Jan 2, 2023 at 13:32
  • @Pᴀᴜʟsᴛᴇʀ2 - There is nothing special in the " see text " section. It only reminds us to slacken the bolts in the reverse order of the tightening. That's it.
    – LFY MP7.3
    Jan 2, 2023 at 18:09
  • These are "torque-to-yield" head bolts? Jan 2, 2023 at 22:56
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    @NarimanAsgharian - That's exactly what they (and I) are stating about the head gasket. Jan 4, 2023 at 14:22
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    @NarimanAsgharian -- yes, or at the very least, that without the preload it may be hard to get a consistent amount of torque (pressure) across the assembly.
    – dlu
    Jan 5, 2023 at 2:49

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