An internal combustion engine has a crankcase, a crankshaft, first and second crankshaft bearings, first and second pistons, first and second piston connecting rods for connecting the first and second pistons to the crankshaft. A mass-balancing system has a counterweight connecting rod pivotally connected to the crankshaft and a counterweight connected to the counterweight connecting rod. The first piston connecting rod is disposed on a first side of the mass-balancing system and adjacent to the mass-balancing system along the crankshaft axis. The second piston connecting rod being is on a second side opposite the first side of the mass-balancing system and adjacent to the mass-balancing system along the crankshaft axis. The first crankshaft bearing is disposed between the first piston connecting rod and the counterweight along the crankshaft axis and the second crankshaft bearing is disposed between the second piston connecting rod and the counterweight along the crankshaft axis.