provided with apertures 50 likewise the inner leg 72 of each arm 60 and 62 of the upper shell 58 is joined by a respective transverse vertically extending leg 78 formed along the outer margin of the central arm 64 the legs 76 and 78 extend toward each other to form one margin for the back leg of each combustion chamber 34 and 36 as best seen in fig5 the other margin for each chamber back leg is formed by one of a pair of vertically upwardly extending spaced apart legs 80 on the bottom shell 56 and one of a pair of respective vertically downwardly extending spaced apart legs 82 on the top shell 58 the legs 80 are joined by a planar rib 84 on the bottom shell 56 and the legs 82 on the top shell are joined by a planar rib 86 engaged with rib 84 to separate the chambers 32 and 34 with each shell defining a pair of parallel u shaped channels communicating with arms 60 and 62 thus the outer side legs 68 and 74 of the bottom and top shells 56 and 58 terminate at positions spaced apart by the ribs 84 and 86 respectively and the separate channels of u shaped cross section for each shell are formed at the central arm 64 for separating the chambers 34 and 36 the ribs 84 and 86 are preferably welded together a peripheral lip 88 is formed at the free or upper edge of side legs.