A cartridge assembly comprising: a housing containing a motor that drives a threaded shaft ; a reciprocally driven piston having a seat end for sealing against a valve seat in a valve body ; a push rod that is threadably engaged to the threaded shaft and driven in an axial direction by rotational movement of the threaded shaft , the push rod coupled to the piston so as to impart said movement in the axial direction to the piston ; an adapter having a first end through which the threaded shaft extends and a second end from which the piston extends, wherein the push rod and the piston are axially moveable relative to the adapter and the adapter seals the housing; a guide press fit into the adapter , wherein at least a portion of the guide extends between the push rod and the piston ; a piston seal for sealing the piston to an interior surface of the adapter in which the piston is slidably guided; an excluder seal between the guide and the push rod , a seal housing that is radially between an inner surface of the adapter and an outer surface of the push rod , wherein a portion of the seal housing is radially outward of the push rod and radially inward of the guide , the seal housing having a recess, that holds a main seal that seals an interface between the push rod and the seal housing , and an outer surface; and a seal element between the outer surface of the seal housing and the adapter , the seal element and the main seal in the seal housing substantially inhibit the passage of fluid into the motor housing and, a bushing press fit to an inner surface of the adapter, the bushing having a bore through which the threaded shaft extends, wherein the seal element seals the bushing to the adapter . a housing containing a motor that drives a threaded shaft ; a reciprocally driven piston having a seat end for sealing against a valve seat in a valve body ; a push rod that is threadably engaged to the threaded shaft and driven in an axial direction by rotational movement of the threaded shaft , the push rod coupled to the piston so as to impart said movement in the axial direction to the piston ; an adapter having a first end through which the threaded shaft extends and a second end from which the piston extends, wherein the push rod and the piston are axially moveable relative to the adapter and the adapter seals the housing; a guide press fit into the adapter , wherein at least a portion of the guide extends between the push rod and the piston ; a piston seal for sealing the piston to an interior surface of the adapter in which the piston is slidably guided; an excluder seal between the guide and the push rod , a seal housing that is radially between an inner surface of the adapter and an outer surface of the push rod , wherein a portion of the seal housing is radially outward of the push rod and radially inward of the guide , the seal housing having a recess, that holds a main seal that seals an interface between the push rod and the seal housing , and an outer surface; and a seal element between the outer surface of the seal housing and the adapter , the seal element and the main seal in the seal housing substantially inhibit the passage of fluid into the motor housing and, a bushing press fit to an inner surface of the adapter, the bushing having a bore through which the threaded shaft extends, wherein the seal element seals the bushing to the adapter . <EOS>