Ferrel tubing is used to space shutters and
to recover expensive concrete shutter tie-rods (Ferrels / Tie Rods).
The inside diameter of the ferrel tubing needs
to be at leat 1mm greater than that of the tie-rod (Ferrel)
being used to fix the shutter. Ferrel tube end cones are
inserted at each end to seal off the hole in the shutter.