Abstract: A casing of a trunk piston internal combustion engine has a cylinder hed, cylinder head cover, cylinder block and oil sump regions and bearing devices for the crankshaft and camshaft. The casing is manufactured from parts which are as simple to manufacture as possible and can be connected together with little effort. For this purpose, the cylinder block, together with a part of the cylinder head closing the respective individual cylinder spaces at the combustion space end, is configured as a common engine block casting with hollow spaces manufactured without the use of lost cores and open in the direction of the cylinder axis at one of the two ends of at least. The hollow spaces are closed by parts welded onto both ends to create a closed continuous water cooling jacket.