Abstract: A rotary valve head assembly including a split head assembly, a rotary spool assembly, spool drive assemblies, and bearing and spool lubrication components. The split head assembly has bores for containing the spool assembly, bearings and spool seals, and defines passages for coolant and lubricant to pass therethrough. The head assembly is separable to provide unencumbered access to the components contained therein. The spool assembly is cylindrical and extends the length of the head assembly. The spool assembly has two ports for each combustion chamber. Each port is provided with a port relief to control the duration it is open. A separate and independent spool drive assembly provides each spool assembly rotating action. Each drive assembly effectively changes the timing of a corresponding spool assembly via instructions from the engine management system, allowing intake and exhaust timing can be controlled independently.