Checking Connecting Rod Bending and Incline

Connecting rods are crucial components in an internal combustion engine, connecting the piston to the crankshaft. They are subjected to tremendous forces and must be perfectly straight and aligned. Any bending or twisting (incline) in a connecting rod can lead to engine damage, excessive wear, and poor performance. Therefore, accurately checking for bending and incline is a vital part of engine rebuilding or inspection.

Understanding the Role of Equipment

Various tools are used in engine diagnostics and component inspection. Let's look at how different equipment might be relevant:

• Surface plate: A highly accurate flat surface used as a reference plane. It's essential for checking the flatness of components, but not directly for measuring the bending or incline of a rod's length or bores relative to each other.
• Dial test indicator (DTI): A sensitive measuring instrument that shows small variations in distance or position. It has a contact point that touches the surface being measured, and a dial that magnifies the movement. DTIs are used for checking runout, alignment, and deviations from a reference point or surface.
• Connecting rod alignment jig: A specialized tool specifically designed to hold a connecting rod and use mandrels (precision pins) inserted through the bores to check for bending and twist, often in conjunction with a dial indicator or feeler gauges.
• Straight edge: A tool with a precisely straight edge, used to check the straightness of surfaces by placing it against them and looking for gaps or using feeler gauges. It's useful for general straightness checks but not precise enough for critical connecting rod alignment.

How a Dial Test Indicator Helps Check Connecting Rods

While a dedicated connecting rod alignment jig is typically used for a comprehensive check, a Dial Test Indicator (DTI) is often a key part of the measurement setup within such a jig or a similar custom fixture. A DTI is used to quantify the amount of bending or twist present.

Here's how a DTI can be used to detect bending and incline:

1. The connecting rod is mounted on a precise fixture or jig that supports it, often using mandrels through the small and big end bores.
2. The fixture provides a reference line or plane.
3. The DTI is set up so its contact point touches a specific point on the connecting rod or a mandrel inserted through one of the bores.
4. By rotating the rod on the mandrel (for twist/incline check) or moving the DTI along the rod's length (for bending check), any deviation from the perfect straightness or alignment will cause the DTI's plunger to move.
5. The dial shows the magnitude of this movement, indicating the amount of bending or incline.

For example, to check for twist (incline), mandrels are inserted through both bores. The rod is mounted on a surface plate or fixture with the small end mandrel resting on supports. The DTI is zeroed on one end of the big end mandrel. The DTI is then moved to the other end of the big end mandrel. If there is twist, the DTI will show a reading, indicating that the big end bore is not parallel to the small end bore. Similarly, for bending, the rod might be supported at the big end, and the small end mandrel's runout checked with a DTI as the big end is rotated, or the DTI might check the rod's beam section against a reference.

Thus, the Dial Test Indicator is the instrument that provides the precise measurement of the deviation, which indicates the bending or incline, often working in conjunction with a suitable fixture or jig.

Revision Table: Connecting Rod Inspection Tools

Additional Information: Importance of Connecting Rod Alignment

Maintaining correct connecting rod alignment is critical for engine health and performance. A bent or twisted connecting rod can cause several problems:

• Increased Bearing Wear: Misalignment puts uneven loads on the connecting rod and crankshaft bearings, leading to premature wear or failure.
• Piston Scuffing: A twisted rod can cause the piston to cock in the cylinder bore, leading to scuffing of the piston skirt and cylinder wall.
• Increased Stress: Bending or twist introduces additional stress on the connecting rod itself, potentially leading to fatigue failure.
• Loss of Performance: Uneven forces and increased friction can reduce engine efficiency and power output.

Therefore, inspecting and correcting any bending or incline using precise tools like a Dial Test Indicator in conjunction with appropriate fixtures is a necessary step in engine maintenance or overhaul.