Rod journal out-of-round is the difference between the journal measurements at:

The correct answer highlights that the rod journal out-of-round is determined by measuring the difference between two specific points on the journal, in this case, E and F. When assessing a journal for roundness, it is crucial to measure it at various points along its surface to identify any deviations from a perfectly circular shape. The measurements at E and F would reflect opposing points on the journal, allowing for an accurate determination of roundness by contrasting these values. This difference provides insight into whether the journal is within specification limits or has developed wear, which is essential for maintaining engine performance and longevity.

In contrast, the other measurement pairs may not adequately represent the entirety of the journal's roundness. Measurements taken from locations that do not represent opposing sides could lead to inaccurate assessments, making understanding of the specific points critical in diagnosing engine issues related to journal roundness.