Plug weld holes should be sized as the vehicle maker recommends, or, if no recommendation exists, what size should be used?

• A. 8 mm (5/16")
• B. 6 mm (1/4")
• C. 10 mm (3/8")
• D. 12 mm (1/2")

Answer: (A)

Plug weld holes must provide enough weld area to develop the joint’s strength without weakening the base metal. If the vehicle maker specifies a hole size, use that exact size. When there’s no spec, eight millimeters (about 5/16 inch) is a common, safe default because it gives adequate weld metal to resist shear loads while preserving surrounding metal. A smaller hole, like six millimeters, might not furnish enough weld area for a strong joint. Larger holes, such as ten or twelve millimeters, remove more metal and increase the risk of burn-through, distortion, or the need for excessive filler.