which clamps between its head 17 and its nut 18 a stack composed in this order of a steel washer 19 a first intermediate washer 20 the lug 9 a second intermediate washer 21 the lug 15 and a nut locking washer 22 each of the intermediate washers 20 and 21 is annular in shape and has an l shaped section comprising an outer leg 23 which is clamped between the washer 19 or the lug 15 and the opposite face 24 25 of the lug 9 and an inner leg 26 substantially adjacent to the wall of the hole 13 the intermediate washers 20 and 21 are produced for example from sintered alloy soaked in oil so as to have a low coefficient of friction and high resistance to compression the lug 7 can be fixed in a conventional manner by virtue of its circular hole 14 when the different parts of the heat exchanger are in thermal equilibrium the bolt 16 ensures the mutual immobilisation of all the components through which it passes and in particular the lugs 9 and 15 the lugs 6 and 8 are immobilised in the same way and the lug 7 in a conventional manner with respect.