This is just the ring piece of the Valley Kayak metric Brass Compression Fitting for the aft end of the Poly-Tube that the steel skeg cable slides in. (Note older rings were symmetric but these new rings will work in the old fittings) The end of the ring with the smaller outside diameter goes toward the end of the poly-tube.

If your problem is that the brass ring at the front end of the poly tube fell off or is slipping, you need to order the "Splice Kit" instead of this ring (the Splice Kit has a brass ring already compressed onto a piece of poly-tube). 

If the poly-tube slipped out of the brass fitting at the aft end of your kayak, it may be best to replace the old brass compression ring (AKA the "olive") when reassembling the system. However, if you are replacing the whole poly-tube, you will need complete brass compression fitting in order to install a brass ring on the front end of the poly-tube. The complete fitting comes with one brass ring, but you may need an extra ring to replace the old one at the aft end of the poly-tube. 

If replacing the whole poly-tube, crimp the compression brass ring onto the front end (the controller end) of the poly-tube then remove both halves of the brass fitting (leaving just the compression ring on the poly-tube). The poly-tube should extend out past the brass ring about 1/4" (look close at the photo for the "Splice Kit" which shows the brass ring in place on the poly-tube). Slide the poly-tube into the kayak from the front end starting on the outside of the control box. Connect all the pieces of the front end before cutting the poly tube to length at the aft end. Allow for the poly tube to have as big a radius as possible where it bends to go into the top of the skeg box. Cut the tube an inch or so long and test fit the tubing into the skeg box fitting before making the final cut. The poly-tube should extend out past the brass ring about 1/4". 

If you are replacing this compression fitting at the back end of the boat (where the poly-tube connects to the skeg box) you will need two 13mm open end wrenches to tighten this compression fitting (one wrench to hold the part of the brass fitting that is screwed into the skeg box and the second wrench to tighten the nut part of the fitting that is on the poly-tube). When tightened, the brass ring is permanently compressed onto the poly-tube (so these rings can't be re-used).

At the aft end you may just need to replace the brass compression ring if the threads and flats for the wrenches are good in the other old parts.

After putting the skeg system together, leak test it by turning the kayak upside down and filling the skeg box with water. Water will flow out the front end of the control system - that is normal. So quickly check inside the hatch for leaks around the top of the skeg box and the poly-tube. Refill the water level as needed. It is normal to have some small leakage (a few drips) even with the compression fitting tightened, but a big leak mean's something didn't get put together right or the compression fitting needs more torque. To finish the job, dry everything and apply liberal amount of Marine Goop all over the compression fitting, the end of the poly-tube and the top of the skeg box where the brass compression fitting screws into the boat.

Tools needed:

Two 13mm open end wrenches

Tubing cutter (to easily make a clean perpendicular cut of the poly tube)