The PCV valve controls the flow of crankcase gases entering the intake system. The airflow through the crankcase and engine interior sweeps away combustion byproduct gases. The block features an aluminium crankcase and cylinder heads. Nonetheless both families have DOHC and an aluminium alloy crankcase and cylinder head. The crankcase and cylinder block had to be made of cast or wrought aluminium alloys. The oil collects in an oil pan, or sump, at the bottom of the crankcase. It used twin flywheels and had an aluminum crankcase. Other changes were mainly in design details, notably a redesigned crankcase and new carburetor. The crankcase and gearbox housing were made of aluminium rather than magnesium for improved reliability. Fuel leaked and entered the crankcase, causing oil dilution. There is normally a bung or stopcock that allows draining of the crankcase before starting. This gives the lightest crank that the crankcase space permits. The propeller was attached to the crankcase (the opposite of radial engines). Day cycle engines are crankcase scavenged and port timed. The first refinement in crankcase ventilation was the "road draft tube".