Why should the crank-connecting rod mechanism be designed into two parts? Isn't integration stronger?

In principle, it is designed like this.

Crank-connecting rod mechanism is a mechanism that transmits engine motion and power. It converts reciprocating linear motion of piston into rotary motion of crankshaft and outputs power. Therefore, the crank-connecting rod mechanism is the main stress component in the engine, and its working reliability determines the working reliability of the engine. With the continuous improvement of engine strengthening index, the working conditions of the mechanism are more complicated. Under the action of various periodically changing loads, how to ensure that the mechanism has sufficient fatigue strength and stiffness and good dynamic and static mechanical characteristics in the design process has become the key problem in the design of crank-connecting rod mechanism. In the traditional design mode, in order to meet the design needs, a large number of numerical calculations must be carried out, and at the same time, in order to meet the service performance of the product, the strength, stiffness, stability and reliability must be designed and checked, and at the same time, the dynamics analysis of the crank-connecting rod mechanism is needed to meet the checking calculation.