Andy's tip about the lighter is also good for fine tuning the grip area on a kydex sheath for a knife as well...but you need to be careful not to burn the kydex...just heat it and then with a glove on...press the lips of the grip area around the handle to whatever level you want for a tight fit. I use this for any sheath which will have an inverted carry....but go in gentle steps and allow to cool and then test it...little increments of bending works best...and keep the sheath away from the flame generously to begin with and again use small increments. Be careful with this because if you burn the kydex you will be "dissapointed". For me though asthetics are not as important as functionality and customising the fit on kydex is something I have had to do for years now to get it "just right". With a Res C handled knife if you want to try this with the knife in the sheath protect the handle with at least tape or better still some fire retardant material. What I use is the fingers cut off a worn out pare of Nomex gloves...the fingers slide over the handle and if you cut out the tip they are basically a "tube" of the right size.
For sheaths too tight I use hot water and immerse the sheath plus knife...handle first into the water to warm it up and enable you to loosen the grip area. Go handle first because the shaped area for the blade can be compromised if the whole sheath is inserted. Use "multiple" dips into the water and keep bringing it out to test the heat level...a couple of withdrawals of the knife from the sheath are usually all it takes.
For knives that are taking a very sharp edge I find that if there is play in the blade part of the sheath the knife will jiggle about when running with it and this will dull the razor edge because of the hardness of the kydex. What I have done is either heat the blade area in water with the knife still in it and let the shaped form fitting open up a bit with the grip area being kept out of the water and used to hold the knife. When the shape opens a bit it should assist with kydex rub and the "slivers" of kydex from withdrawing the knife.
If you then still get a bit of loose rattle with the knife...get an old pedal bike inner tube and cut a strip and fold it over the edge so it can act as an edge protector as it is much softer than the kydex. All kydex should have a hole at the bottom to enable water to drain away...invert the sheath and drop a threaded needle through the hole...collect it at the open end and push the needle through the rubber inner tube cut out to protect the edge...I usually just cut a section which will protect the area from the tip round to the belly curve and a little way up the flat part of the edge...use the thread to then pull it into place inside the sheath whilst wrapped around the knife edge and using both the benefit of pushing in with the knife and being able to pull with the thread you can get the rubber to nicely push down to the base of the sheath...it then acts as a cushion for the blade and the edge...and so far all my knives with a very sharp edge are able to be carried without edge damage. The thread is then easy to pull away if you have dropped the needle back down the sheath so it was doubled over...you just need to pull one side.
It might all sound a bit "heath robinson" but these tips do work...I did all of this for my RMD sheath and can carry this knife with the sharpest of edges no problem....ditto on my Jackhammer...FSH's...DFLE...and FBM.
