Ordered such things twice from Aliexpress:
- in one set (5x2 wires, with slightly smaller alligator jaws), the cover is still soft, yet they became greasy-slippery only on the inside. They are hard to open without the alligator inside flipping sideways.
- in the second set of 10 wires, the cover hardened, so they can't fully-open, nor fully-close
Aside of the plastic cover BEWARE that:
- the wire looks like copper, but it is not copper (easy to test with a magnet). That piece of wire, as short as it is, has about 1 ohm resistance

, and heats a lot under high current, can start a fire if used above 3-5A or so
- even worst, the 1 ohm wire are not properly crimped, they make a very bad contact between alligator and wire
Replace the original wires with copper ones, and solder the wire to the alligator, because the metal sheet from which the alligator was made is too thin and brittle to crimp a wire.
Even when in good shape, the contact resistance of such alligators is rather large and not constant. If you clean them with IPA might do much better for a while (this is true to any chrome finish surface, in time they tend to cover themselves with an invisible layer of grease molecules captured from air).
The metal sheet of the alligator is flimsy, which makes their bite not uniform.
As for the plastic isolation
- try cutting the cover between the opened jaws, similar in the back of the jaws, such that the cover stays uncut in the middle, where the jaws axis is, or
- replace the isolation with some other tube, I've tried once some silicon tube (food-grade transparent tube), but you have to have just the right diameter, and it was not working very well. At least that has high melting temperature and doesn't burn like PVC
- or maybe paint the metal on the outside with some insulator lacquer, like nail-polish? (I didn't try this last one)