To easily fit auxiliary units onto a side flange by fitting adaptor plates superposed on the openings of the auxiliary units to the end face of the side flange, and working the surface of the plates, so that the auxiliary units to be fixed here can be engaged therewith.

This engine block 10 of an engine for tractor is integrally formed with reinforcing ribs 18, 20 extending up to a side flange 12, and a fuel pump 14 and an hydraulic pump 16 as auxiliary unit are mounted on the side flange. In this case, the side flange 12 are castingly formed with openings of large diameter for each auxiliary unit 14, 16, and the respective openings are formed into sizes so that the whole bodies of the respective fitting plates of the respective auxiliary units 14, 16 can be inserted through. Adaptor plates 30, 32 formed into larger diameters so as to blockade the respective openings are arranged, they are fixed to the rear face of the side flange 12 with bolts 34, 32, and then the respective auxiliary units 14, 15 are fixed to the respective plates 30, 32.