Rotary valve assemblies are disclosed, for example, for use in a thermal management system. The assembly may include a first rotary valve including a primary shaft including a hollow portion defining a first inner cavity and a first spline piece disposed within the first inner cavity on one end and including a primary spline feature on an opposite end, the primary spline feature having one or more primary splines. A second rotary valve may include a secondary shaft including a hollow portion defining a second inner cavity and a second spline piece disposed within the second inner cavity and including a secondary spline feature having one or more secondary splines. One or both of the first spline piece and the second spline piece may be formed of metal. The primary splines and the secondary splines may be configured to mesh together to form a spline joint between the rotary valves.