The 12 valve, mechanical Cummins 5.9 engine is very popular in school buses between the late 1980s and the late 1990s. Originally introduced in 1984 for agricultural uses, it earned a solid reputation for reliability and longevity. This engine was also very popular in Dodge pickup trucks for both its stock performance and the potential to get incredible performance upgrades with very little effort.
The family consisted of the 6B5.9 (Naturally Aspirated), 6BT5.9 (Turbocharged), and 6BTA5.9 (Turbocharged + Aftercooled).
Due to its popularity, parts availability for these motors is still very good. Many parts are shared with the proceeding generation, the 24 valve ISB 5.9. Physical differences between truck and bus versions are few, notably including use of an intake heater to aid in cold weather starting.
The entire range is covered in the operation and maintenance manual in quite good detail. You can download the manual here:
Cummins B Series Operation & Maintenance 3810205-14