What are the faint lines visible on my KMB 28 frame, and do they indicate a structural crack?

The lines you see are known as “knit lines” or “mold flow lines,” which are a standard characteristic of the injection molding process. They form when molten plastic flows around an internal object (such as a threaded insert) and rejoins on the other side.

While they may resemble cracks, they are strictly surface-level marks, not actual fractures. These lines have been present since the original KMB 10 frames were produced because the tooling reflects engineering standards from approximately 50 years ago—before modern digital flow analysis was available. Despite their appearance, they are not points of weakness; in five decades of use, these robust, thick-walled frames have never experienced a structural failure related to these flow lines.