Die collar:N: a collar or coupling of tool steel, threaded internally, that can be used to retrieve pipe from the well on fishing jobs; the female counterpart of a taper tap.
The die collar is made up on the drill pipe and lowered into the hole until it contacts the lost pipe.
If the lost pipe is stuck so that it cannot rotate, rotation of the die collar on top of the pipe cuts threads on the outside of the pipe, providing a firm attachment.
The pipe is then retrieved from the hole.
Compare taper tap.
It is not often used because it is difficult to release it from the fish should it become necessary.