accommodating the bolts 42 43 the boiler plates 44 45 are held together both by a bottom sheet metal ring 47 and a top sheet metal ring 48 the bottom sheet metal ring 47 has an outer circumference that corresponds to the outer circumference of the outer boiler plate 45 the bottom sheet metal ring 47 therefore is flush on the outside with the outer boiler plate 45 an inside diameter of the bottom sheet metal ring 47 is slightly larger than the internal thread diameter of the thread insert 41 such that the thread insert 41 rests on the bottom sheet metal ring 47 toward the inside the inner boiler plate 44 has guiding ribs which are not shown in the drawing complementary to these guiding ribs an outer wall of the thread insert 41 has guide grooves extending in an axial direction that is to say in the direction of the axis 49 of fig6 this complementary connection represents an anti rotation locking mechanism of the thread insert 41 relative to the thread carrier 40 about the axis of rotation 49 the top sheet metal ring 48 has an outside diameter that corresponds to the outside diameter of the outer boiler plate 45 the top sheet metal ring 48 therefore is flush toward the outside with the outer boiler plate 45 the top sheet metal ring 48.