The thread cutting diestock assembly comprises a ratchet housing (20) having a bore (22) adapted to selectively and detachably receive a large thread cutting die head or an adaptor ring (12), each of which is provided with ratchet teeth (44) engaged by a the end (38) of a pawl on the ratchet housing (20) so as to be rotatable therewith. The adaptor ring (12) has a polygonal bore (48) adapted to detachably receive a small thread cutting die head (14) which interengages with the polygonal bore (48) to preclude relative rotation between the die head (14) and adaptor ring(12). A circumferentially extending recess (42) in the bore (22) in the ratchet housing (20) carries a non-circular split retaining spring ring (16) which interengages with the large die head or adaptor ring (12) to detachably secure the latter in the ratchet housing bore (22), and a circumferentially extending groove (58) in the adaptor ring bore (48) carries a polygonal split retaining spring ring (18) which interengages with the smaller die head (14) to detachably secure the latter in the adaptor ring bore (48).
