reference is made first of all to fig1 which shows a heat exchanger 10 comprising a header tank 12 according to the invention , into which the ends of the tubes 14 of a heat exchanger bundle open . the tubes 14 are parallel flat tubes between which are positioned corrugated inserts 16 that form heat exchange fins . the heat exchanger 10 comprises an opposite header tank ( not depicted ) analogous to the header tank 12 . the exchanger 10 finds a particular application in motor vehicles and is designed to be produced by brazing in a single operation while at the same time offering good ability to withstand pressure and a high yield strength . as can be seen in the sectioned view of fig2 , the header tank 12 is made up of two metal components nested one inside the other intended to be assembled by brazing . the header tank 12 comprises a manifold 18 produced in the form of a metal element ( for example made of aluminum or an aluminum alloy ) with a u - shaped profile having a bottom wall or plate 20 equipped with holes 22 for the insertion of tubes 14 . the manifold also has two lateral walls or lateral edges 24 connected to the plate 20 . the lateral walls 24 of the manifold 18 that form the two branches of the u are of identical heights , although it is possible for the height to vary relative to the bottom wall 20 . the plate 20 is generally flat and the same is true of the lateral edges 24 . these meet the plate 20 at an angle of about 90 °. this angle could be different . the plate 20 is connected to the lateral edges 24 via curved portions 26 . as can be seen in fig1 and also in fig3 , the lateral edges 24 have a variable height near the end of the manifold 18 . in particular , the lateral edges 24 of the manifold have two curved portions 28 each of which extends as far as the plate 20 . the header tank 12 comprises another piece , namely a cover 30 made in the form of a metal element ( for example of aluminum or an aluminum alloy ) with a u - shaped profile having a bottom wall 32 extending in a plane parallel to the plane in which the plate 20 extends , and two lateral walls 34 connected to the bottom wall 32 . the lateral edges 34 of the cover are generally flat and make an angle of about 90 ° with the bottom wall 32 . as depicted in fig2 , the lateral walls 34 of the cover 30 that form the two branches of the u may be of identical heights . the cover 30 is nested inside the manifold 18 in such a way that their respective u - shaped profiles face in the same direction , that is to say away from the tubes 14 of the heat exchanger bundle . the result of this is that the lateral walls 34 of the cover 30 come into contact , via their outside , with part of the lateral edges 24 of the manifold 18 , on the inside of the latter ( fig2 ). the cover 30 further comprises two end regions 36 ( fig3 ) each of which is curved toward the plate 20 of the manifold 18 . in each of these end portions , the bottom wall 32 of the cover has a curved portion 38 ending in a rim 40 capable of pressing against the plate 20 of the manifold 18 . the rim 40 is itself connected to a bent part 42 of each lateral edge of the cover to close the header tank 12 . large surface areas with good surface contact that are highly suitable for brazing can thus be brazed together . the manifold 18 is advantageously produced by bending a metal sheet such that it can be given a u - shaped profile , the holes 22 intended for the insertion of the tubes 14 preferably being produced in the metal component after bending . the cover 30 for its part is advantageously produced by pressing , the end portions of the cover also being produced during this pressing operation . to strengthen this header tank , ribs are made in the manifold 18 . fig4 is a perspective part view of the header tank 12 providing a better illustration of how the ribs are distributed . prior to bending , the manifold 18 is pressed to form three distinct types of rib in the plate 20 , the lateral edges 24 and the curved portions 26 . ribs 50 are first of all produced between each of the holes 22 . the ribs 50 extend over the entire width of the plate 20 between the curved portions 26 and the lateral edges 24 . the ribs 50 thus extend uniformly over the length of the manifold 18 . the ribs 50 have a substantially v - shaped cross section and are produced toward the outside of the tank 12 . the ribs 50 thus have a convexity extending substantially in the opposite direction to the direction in which the lateral edges extend . the ribs 50 thus form projections surrounding the holes 22 . each pair of ribs 50 surrounding a given hole 22 forms a guide for the insertion of the tubes 14 , each by way of one branch of the v of which it is formed . other types of cross section for the ribs 50 are conceivable , so as to allow even better insertion of the tubes 14 while at the same time providing the manifold 18 with optimum rigidity . ribs 52 are produced in the curved portions 26 . like the ribs 50 , the ribs 52 extend in the direction of the width of the plate 20 and are each positioned facing a rib 50 in the continuation thereof . the ribs 52 are produced toward the inside of the tank 12 . the ribs 52 thus have a convexity extending substantially in the direction in which the lateral edges 24 extend . a region 54 allows each rib 50 to be connected to the rib 52 which is its continuation and which departs in substantially the opposite direction . the ribs 52 allow for a substantial improvement in the ability to withstand pressure and in the yield strength . this aspect is all the more critical because , traditionally , the portions 26 have constituted significant weak points in header tanks of the type that the tank 12 represents . finally , ribs 56 are produced in the lateral edges 24 . like the ribs 50 and 52 , the ribs 56 extend in the direction of the width of the plate 20 and are each positioned facing a rib 52 , in the continuation thereof . the ribs 56 are also produced toward the inside of the tank 12 . the ribs 56 thus have a convexity extending substantially in the direction of the opposite lateral edge . the ribs 56 extend over most of the lateral edges 24 , up to a chosen distance away from the end of the edges 24 that will allow the cover 30 to be housed and brazed into the manifold 18 . in the example described here , the ribs 50 , 52 and 56 are not only in the continuation of one another but are also continuous . these ribs thus form a rib which extends over the entire width of the manifold 18 , from a position close to the end of one lateral edge to another position close to the end of the opposite lateral edge , passing via the plate . furthermore , while all the holes 22 depicted are flanked by ribs 50 , 52 and 56 , the invention does not exclude the situation whereby the holes 22 at the ends of the plate 20 are flanked by just one rib 50 , 52 or 56 , or none at all . in another embodiment depicted in fig5 , the ribs 50 and 52 are unchanged in their constitution and layout . by contrast , the ribs 56 are offset , and each positioned facing a hole 22 , that is to say such that they alternate with a rib 52 . the invention is not restricted to heat exchangers the header tanks of which are made of aluminum or aluminum alloy . it may in particular be applied to header tanks with the plate and the cover made of plastic , ribs then being produced for example at the groove in the manifold plate and between the holes that this comprises . furthermore , a person skilled in the art will be able to conceive of all the alternative forms that arise out of studying the claims that follow .