Yes expansion and header on same circuit, runs unpressurized mainly because I didn't have a suitable tank and also didn't want very hot water under pressure in radiators, not necessary just my choice.
Bypass valve is manual, if we want engine preheat, valve is positioned to divert hot Mikuni coolant through engine then returned to original position when engine is started, engine them carries on circulating coolant with assistance of Mikuni pump. When coolant reaches a certain temp Mikuni cuts out leaving it's pump running, if Mikuni isn't used for preheat then engine coolant pump does the circulating.
One drawback is the engine takes longer to warm up as it is circulating coolant through calorifier and/or rads + Mikuni though valves can be set so that engine is isolated from Mikuni and rads but coolant can still circulate as normal.
As in previous post we run it un-pressurized but that is just my choice.