Plug weld holes should be sized to 8 mm (5/16") when no vehicle maker recommendation exists.

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

Answer: (A)

When a vehicle maker doesn’t specify a plug weld hole size, the goal is to choose a size that provides enough weld area for a strong join without causing heat damage to thin body panels. An 8 mm (5/16 in) hole offers a practical balance: it creates enough weld metal to fuse the panels under typical load without requiring excessive heat input. A smaller hole, like 6 mm, may not provide enough weld area for adequate strength. Larger holes, such as 10 or 12 mm, increase heat input and the risk of burn-through, warping, or misalignment. So 8 mm is the standard default that tends to work well across common sheet thicknesses when there’s no manufacturer guidance.