This is a CNC machined flexible coupling that is ideal for connecting the Z-axis screws to the stepper motors on a Prusa-Mendel Reprap type 3D Printer
The 5mm bore is for the output shaft of a NEMA17 stepper motor and the 8mm bore is for the M8 threaded rod.
The actual size of M8 threaded rod is usually only about 7.8mm but this coupling will close down enough to grip it tightly.
The coupling is made of Aluminium.
Diameter = 20mm
Length = 25mm
Weight = 17g
The price is for 1 coupling.