What would be indicated by a grinding noise when trying to shift a five-speed synchronized transmission from 4th to 5th gear?

A grinding noise when shifting from 4th to 5th gear in a synchronized transmission typically suggests an issue with the blocking rings. These rings are essential for smoothly engaging and disengaging gears; they help to achieve the proper synchronization of speeds between the gears before engagement. When the blocking rings wear down, they may not effectively match the speeds of the gear and the main shaft during shifts. This mismatch causes the grinding noise because the gear isn’t properly synchronized before it tries to engage, resulting in the teeth clashing.

In this case, while worn main drive gear teeth and worn countershaft drive gear teeth can also lead to problems in transmission performance, they generally manifest in different symptoms and would not specifically cause a grinding sound during a shift. Similarly, insufficient clutch free pedal travel indicates a problem related to the clutch system that typically results in difficulty disengaging or engaging the gears rather than the grinding sound specifically. Thus, the grinding noise is most directly linked to the condition of the blocking rings.