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: