Plastigage: The Indispensable Tool for Precise Bearing Clearance Measurement

Introduction

Plastigage is a remarkable tool that revolutionized the way bearing clearances are measured in mechanical assemblies. Its simple yet effective design and application have made it a staple in the toolkits of engineers, mechanics, and DIY enthusiasts alike. This comprehensive guide will delve into the world of plastigage, exploring its benefits, applications, techniques, and the fascinating stories and lessons it has to teach.

Understanding Plastigage

Plastigage is a semi-solid, non-hardening measuring compound made from a blend of viscous polymers and color-coded indicators. When a thin strip of plastigage is placed between two mating surfaces subjected to pressure, it flattens and its width changes. This change in width directly corresponds to the clearance between the surfaces.

Why Plastigage Matters

Measuring bearing clearance is crucial for ensuring proper operation and longevity of mechanical assemblies. Excessive clearance can lead to increased friction, noise, and wear, while insufficient clearance can cause binding and premature failure. Plastigage provides a quick, accurate, and non-destructive method for determining the ideal clearance, preventing costly repairs and downtime.

Benefits of Using Plastigage

Accuracy: Plastigage has an accuracy of ±0.001 inches (±0.025 mm), making it ideal for precise clearance measurements in various applications, including engines, transmissions, and bearings.

Ease of Use: Plastigage is extremely user-friendly. Its simple application and straightforward interpretation eliminate the need for complex tools or extensive training.

Non-Destructive: Unlike other clearance measurement methods, plastigage does not require disassembly or modification of the assembly, preserving its integrity and functionality.

Wide Applicability: Plastigage is suitable for measuring clearances in a wide range of materials, including metals, plastics, and composites, making it a versatile tool for various industries and applications.

Cost-Effective: Plastigage is an affordable and cost-effective solution for measuring bearing clearances compared to other methods that require specialized equipment or professional services.

Applications of Plastigage

Plastigage finds applications in numerous industries and fields, including:

Automotive: Adjusting main and connecting rod bearing clearances in engines.
Manufacturing: Measuring clearances in gears, bearings, and other machined components.
Aerospace: Setting clearances in aircraft engines and landing gears.
Industrial Machinery: Determining clearances in pumps, compressors, and other rotating equipment.
DIY Projects: Ensuring proper clearances in engine rebuilds, small appliance repairs, and custom fabrications.

Techniques for Using Plastigage

Surface Preparation: Clean and degrease the mating surfaces thoroughly to ensure accurate results.

Plastigage Placement: Place a thin strip of plastigage along the entire length of the clearance to be measured.

Assembly and Measurement: Assemble the components and tighten them to the specified torque or force.

Disassembly and Interpretation: Disassemble the assembly and carefully remove the flattened plastigage strip. Compare the flattened width to the reference chart provided with the plastigage to determine the clearance.

Stories and Lessons from Plastigage

Story 1: The Overlooked Clearance

A mechanic was tasked with rebuilding an engine. He meticulously measured all the bearing clearances using plastigage except for one connecting rod bearing. Assuming it would be similar to the others, he assembled the engine and ran it. However, a persistent knocking sound developed, indicative of excessive bearing clearance. Upon disassembly, he discovered that the overlooked bearing had a much larger clearance, causing the knocking. This experience taught him the importance of measuring every clearance accurately, no matter how small or seemingly insignificant.

**Story 2: The Misleading Clearance

A DIY enthusiast was rebuilding an old motorcycle engine. He used plastigage to measure the main bearing clearances and was satisfied with the results. However, after assembling the engine, it seized up due to insufficient oil flow. Upon further inspection, he realized that the plastigage had squeezed out to the sides during assembly, giving a false indication of adequate clearance. This lesson emphasized the importance of ensuring proper plastigage placement and considering the effects of assembly forces.

**Story 3: The Precision Fix

A skilled machinist was tasked with repairing a precision milling machine that had developed excessive backlash in its spindle bearings. Using plastigage, he meticulously measured the clearances and adjusted the bearings accordingly. The machine was restored to its original precision, demonstrating the effectiveness of plastigage in solving complex clearance issues.

Tips and Tricks

• Use the correct plastigage grade for the clearance range you are measuring.
• Apply plastigage sparingly to avoid excessive squeezing or interference with assembly.
• Protect plastigage from contamination, as dirt or grit can affect accuracy.
• Use a magnifying glass or a smartphone camera with macro mode to enhance the visibility of the flattened plastigage.
• If the plastigage is too thin or too thick, it may not provide accurate results.
• Store plastigage in a cool, dry place to maintain its accuracy and shelf life.

Conclusion

Plastigage is an indispensable tool for precise bearing clearance measurement. Its ease of use, non-destructive nature, and wide applicability make it a valuable asset for engineers, mechanics, DIY enthusiasts, and anyone working with mechanical assemblies. By understanding the benefits, applications, techniques, and lessons from plastigage, you can ensure the proper functioning and longevity of your mechanical equipment.