For plug welds when no vehicle maker recommendation exists, what hole size should be drilled or punched in the location of the spot welds being replaced?

• A. 8 mm
• B. 5/16 inch
• C. 3/16 inch
• D. 1/4 inch

Answer: (B)

When you replace a spot weld with a plug weld, you drill or punch a hole at the weld location so the plug weld can fuse the patch to the base metal. If there’s no maker’s spec to follow, you use a standard default size that provides enough weld area without risking damage from heat. The commonly taught default is five-sixteenths of an inch, which is about eight millimeters. This diameter gives sufficient cross-sectional area for a strong plug weld across typical panel thicknesses, while keeping heat input under control to minimize burn-through. Smaller holes reduce the weld area and strength, and larger holes increase heat input and the risk of damaging the panels or causing distortion. The 8 mm size is essentially the same diameter, but the standard reference used in practice is five-sixteenths of an inch.