Sort of all ready stated but but in the old days when bolts were only torqued and with the torque and tighten x degrees the idea is to stretch the bolt a certain amount.

I have seen on other applications where the Engine has Studs instead of Bolts (and the manual calls for it) you would use a Dial Idicator to measure the elongation/length of the Stud as you tighten it. But, the idea is the same; you are stretching it to a certain amount.

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84 300D, 82 Volvo 244Gl Diesel