The crankshaft connecting rod is a critical transmission component in two-stroke gasoline engines, responsible for converting the reciprocating linear motion of the piston into the rotational motion of the crankshaft. Its positioning accuracy directly affects the engine’s operational stability, power output, and service life. The connecting rod consists of two key parts: the big end (crank end) connected to the crankshaft and the small end (piston end) connected to the piston. Although both ends serve the purpose of positioning and motion transmission, they differ significantly in positioning objectives, structural designs, technical requirements, and functional priorities due to their distinct working environments and force-bearing conditions. This article elaborates on the core differences between the big end and small end positioning of the crankshaft connecting rod in two-stroke gasoline engines.

1. Differences in Positioning Objectives

The fundamental objective of positioning is to ensure precise motion transmission and structural stability, but the focus varies significantly between the big end and small end due to their different connection objects and motion characteristics.

The big end positioning is primarily aimed at achieving a stable and rotatable connection with the crankshaft’s crankpin, ensuring that the connecting rod can smoothly rotate around the crankpin while transmitting the torque generated by the piston’s linear motion to the crankshaft. In two-stroke gasoline engines, the crankshaft rotates at high speed, and the big end bears periodic alternating loads (tension and compression) and centrifugal forces. Therefore, its positioning must prioritize rotational flexibility and load-bearing stability to avoid jamming, wear, or even structural failure during high-speed operation. Additionally, the big end positioning needs to ensure the axial and radial alignment of the connecting rod relative to the crankshaft, preventing lateral displacement that could cause uneven wear of the crankpin and bearing surfaces.

In contrast, the small end positioning focuses on forming a flexible pivot connection with the piston’s gudgeon pin (wrist pin), allowing the connecting rod to swing freely around the gudgeon pin as the piston reciprocates up and down in the cylinder. The core requirement here is to maintain the coaxiality of the small end, gudgeon pin, and piston, ensuring that the piston moves smoothly along the cylinder axis without tilting or jamming. Since the small end moves with the piston in a reciprocating manner, its positioning must also accommodate the changes in the connecting rod’s angle during motion, minimizing frictional resistance and ensuring the consistency of the piston’s movement trajectory.

2. Differences in Structural Design and Positioning Methods

The structural design of the big end and small end is tailored to their respective positioning objectives and working conditions, resulting in distinct positioning methods and component configurations.

2.1 Big End Positioning Structure

The big end of the connecting rod is a larger, split-type structure (composed of the rod body and a detachable cap) to facilitate assembly and disassembly with the crankshaft crankpin. This split design allows the big end to be bolted around the crankpin without disassembling the entire crankshaft, which is crucial for engine maintenance and assembly. The inner surface of the big end is equipped with a bearing (typically a plain bearing or roller bearing) that directly contacts the crankpin, reducing frictional resistance during rotation.

Positioning of the big end is achieved through multiple mechanisms: first, the bolted connection between the rod body and the cap ensures that the two halves of the big end are tightly clamped around the crankpin, maintaining radial positioning accuracy. Second, the bearing shells inside the big end are equipped with positioning lips that fit into the grooves of the big end, preventing the bearing shells from rotating or moving axially due to friction. In many two-stroke engines, shims are also installed on both sides of the big end bearing to center the connecting rod and prevent friction between the big end and the crankshaft webs. Additionally, the center of the big end’s bearing hole must be precisely aligned with the center of the crankpin to ensure smooth rotation and uniform load distribution.

2.2 Small End Positioning Structure

The small end of the connecting rod is a smaller, integral structure (non-split) with a through hole that accommodates the gudgeon pin. Unlike the big end, the small end does not require disassembly for assembly, as the gudgeon pin is inserted through the small end hole and the piston pin holes, securing the connection between the connecting rod and the piston. The inner surface of the small end is usually equipped with a bushing (such as a phosphor bronze bushing) or a needle roller bearing, which reduces friction between the small end and the gudgeon pin during reciprocating swing.

Positioning of the small end is mainly dependent on the fit between the gudgeon pin and the small end hole, as well as the fit between the gudgeon pin and the piston pin holes. The gudgeon pin is typically secured using circlips installed at both ends of the piston pin holes, preventing axial movement of the pin and ensuring that the small end remains aligned with the piston. In some early small-capacity two-stroke engines, shims were used on the gudgeon pin to center the connecting rod, but modern designs often rely on precise machining of the small end hole and gudgeon pin to achieve accurate positioning. The center of the small end’s hole must be aligned with the center of the gudgeon pin to ensure that the connecting rod swings smoothly without causing lateral forces on the piston.

3. Differences in Technical Requirements and Tolerances

Due to the different force-bearing conditions and motion characteristics, the big end and small end positioning have distinct technical requirements, particularly in terms of dimensional accuracy, fit clearance, and surface roughness.

The big end positioning requires higher dimensional accuracy for the bearing hole diameter and the distance between the big end and small end centers (center-to-center distance). The center-to-center distance is a critical parameter that affects the engine’s stroke and rod ratio, which in turn influences piston acceleration, combustion efficiency, and engine smoothness. The fit clearance between the big end bearing and the crankpin is strictly controlled: excessive clearance can cause vibration and noise, while insufficient clearance can lead to insufficient lubrication and overheating. Additionally, the surface roughness of the big end bearing and crankpin contact surfaces must be very low (typically Ra ≤ 0.8 μm) to reduce friction and wear. The bolt torque for the big end cap is also a key technical requirement, as insufficient torque can cause the cap to loosen, while excessive torque can damage the bolts or deform the bearing hole.

For the small end positioning, the key technical requirements focus on the coaxiality of the small end hole and the gudgeon pin, as well as the fit clearance between them. The small end hole must be precisely machined to ensure that its axis is parallel to the big end hole axis, preventing the connecting rod from tilting during motion. The fit clearance between the small end bushing (or needle bearing) and the gudgeon pin is slightly larger than that of the big end, as it needs to accommodate the reciprocating swing of the connecting rod and the thermal expansion of components during engine operation. The surface roughness of the small end hole and gudgeon pin is also strictly controlled, but the tolerance requirements for the center-to-center distance are less stringent than those of the big end, as the small end’s motion is primarily guided by the piston.

4. Differences in Load-Bearing Capacity and Wear Resistance

The big end and small end bear different types and magnitudes of loads, leading to differences in their load-bearing capacity and wear resistance requirements.

The big end bears significantly higher loads than the small end. During the engine’s combustion stroke, the high-pressure gas pushes the piston downward, generating a large linear force that is transmitted to the big end through the connecting rod. This force, combined with the centrifugal force generated by the crankshaft’s high-speed rotation, creates alternating tension and compression loads on the big end. To withstand these loads, the big end is usually made of high-strength steel (such as 4340 chromium-molybdenum steel) through drop forging or die forging, ensuring high tensile strength and fatigue resistance. The bearing shells of the big end are often lined with soft metals (such as tin or babbitt) to improve wear resistance and reduce friction, and some modern two-stroke engines use roller bearings at the big end to better cope with harsh operating conditions where lubrication may be intermittent.

The small end bears relatively smaller loads, primarily the lateral forces generated by the piston’s reciprocating motion and the bending forces from the connecting rod’s swing. As a result, the small end can be made of lighter materials (such as aluminum alloys for low-load applications) or the same high-strength steel as the big end, depending on the engine’s power output. The small end bushing or needle bearing is designed to withstand repeated oscillating motion, and the use of needle bearings in modern two-stroke engines has significantly improved the small end’s wear resistance and service life. Early small-capacity two-stroke engines often used plain phosphor bronze bushes in the small end, which are more sensitive to poor lubrication and have a lower maximum rev limit.