A crankshaft includes first, second, third, and fourth main journals, each coaxially disposed along a crankshaft axis. A first, second, third, fourth, fifth, and sixth crank pin are each parallel with the crankshaft axis, and radially offset from the crankshaft axis. A first crank arm includes a first counterweight, and connects the first main journal and the first crank pin. A third crank arm includes a second counterweight, and connects the second crank pin and the second main journal. A seventh crank arm includes a third counterweight, and connects the third main journal and the fifth crank pin. A ninth crank arm includes a fourth counterweight, and connects the sixth crank pin and the fourth main journal. The first, second, third, and fourth counterweights are the only counterweights of the crankshaft.