Which of the following practices is not allowed in top rails?

Patching is not permitted on top rails due to the critical nature of structural integrity in that area of a dry van. The top rail serves as a primary structural component, providing necessary strength and rigidity to the trailer.

Inserting, sectioning, and welding are methods that might be employed for repairs or adjustments. Inserting can involve placing additional material to enhance strength, sectioning refers to cutting out and replacing portions of the rail to maintain or restore its structural capabilities, and welding can effectively join materials together to provide solid connections. However, patching, which typically involves filling in a hole or defect, does not restore strength adequately and can lead to failure under stress, making it an unacceptable practice for top rails where safety and durability are paramount.

Maintaining the integrity of the top rail is essential to uphold safety regulations and support the overall structural framework of the dry van.