The block (B) comprises a piston hub having a horizontal axis (X) and housing a piston and a shaft hub housing a crankshaft and having a vertical axis (Y) intersecting the horizontal axis (X). The block (B) incorporates a connecting portion having a first end attached to a region of the piston hub disposed on a side of the horizontal axis (X) opposite to that turned to the shaft hub, and a second end attached to an adjacent end portion of the shaft hub. The connecting portion is elastically deformable by a resulting bending moment (MF) generated: by a first compression derived force (F1) actuating on the second end of the connecting portion; and by a second compression derived force (F2) applied to a free end portion of the shaft hub and which tends to provoke an angular displacement of the vertical axis (Y) of the shaft hub in the direction of the first compression derived force (F1).