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.