When preparing to fit thick-wall boiler tubing, which measurement must be matched?

• A. Outer diameter
• B. Inside diameter
• C. Wall thickness
• D. Length

Answer: (B)

The important idea is matching the bore you’ll work with—the inside diameter. When fitting thick-wall boiler tubing, the internal opening determines how the tube will mate with any insert, bushing, or fitting and how the flow path will align. If the inside diameter doesn’t match the mating part, you’ll get poor fit, leaks, or misalignment, even if the outer surface and wall thickness seem correct. Outer diameter and wall thickness affect strength and external fit, but the internal bore size governs how the tube fits with components that go inside or connect through it, so it must be matched.