Butterfly knives operate through hinges in the handle that connect to the blade. Over time, the hinge-pins can wear out making the operation of your butterfly knife sloppy and loose. Some butterfly knives use screws for the hinges, allowing you to adjust the tightness; others use pins, which make for smoother operation, but when worn out, need to be replaced. This can be done using a few commonly found materials and a trip to the local hardware store.
Place the tip of the nail over the hinge-pin on one of the two handles of the knife. Do this on a soft wooden surface that you do not mind damaging.
Hit the nail with the hammer. This will drive the hinge-pin out the other side. Turn the knife over and pull out the pin, using the pliers. Do this to the other side as well.
Go to your local hardware store and measure the diameter of the pin. Purchase a replacement rod of this diameter.
Cut the rod down to the appropriate length of the hinge-pin using the hacksaw.
Hammer the rod pin from Step 4 into the handle, through the blade and secure it into the other side of the handle by tapping the pin flush on both sides. Do this for the other handle as well.
Butterfly knives are illegal in certain areas. Check with your local authorities before acquiring or fixing one.