An engine exhibits worn valve guides shortly after an overhaul. Which could be an exception to the causes of excessive guide wear?

The option regarding stem-to-guide clearance slightly beyond normal tolerance can be considered an exception to the causes of excessive guide wear because it doesn't necessarily indicate a direct cause of rapid wear. While it is true that clearance beyond the normal specification may lead to some wear, it is often considered acceptable within a small margin. As such, this factor is not as immediately impactful as the others listed.

Improper lubrication, such as leaving the OEM seal, can result in inadequate oil delivery to the valve guides, leading to increased friction and wear. Carbon build-up can also contribute significantly to wear by creating abrasive particles, which further aggravate the condition of the guides. Valve springs that are out-of-square can apply uneven forces on the valves, causing them to wear unevenly and potentially accelerating wear on the guides. Thus, while the guide clearance can influence wear levels, it is less likely to produce excessive wear in the same way that the other factors would.