An instrument holder assembly for use with a robotic surgical system can comprise one or more modules for holding or guiding an instrument. Each module can comprise a variable diameter jaw having movable teeth configured to hold or guide different sized instruments. Each module can be attached to opposite ends of a main shaft that can couple to an arm of the robotic surgical system. A central passage can extend through each module and the main shaft and the variable diameter jaws can constrict the central passage. Teeth of the variable diameter jaws can be biased to a closed or opened position. A tensioner tool can be used to hold biasing elements in position so that the teeth can be loaded into the module. The teeth can be loaded into slots of the module using a tooth holder device.