Abstract: An improved desilter for drilling muds consists of one or more hydrocyclones with certain features providing for better separation of large particles from the mud. Drilling muds used in drilling oil wells and the like give substantially improved performance when the larger particles are removed therefrom and the mud consists of fine particles having a narrow range of particle size.An improved hydrocyclone for removing coarser particles from the drilling mud has overflow and underflow outlets which discharge openly rather than into a collection manifold. The overflow outlet is in the form of a controllable syphon which may be adjusted in syphon length or which may include means to adjust the effective length of the syphon leg. The adjustment of the syphon leg controls the overflow of the thin, more uniform mud and effects a superior separation of the larger particles from the mud.