A process is disclosed for inserting a piston pin wire lock ring, particularly Circlips, into a locking groove of a wrist pin bore of a piston. The wire lock ring is radially pre-compressed and thus reduced in size in such a manner that its outside diameter is smaller than the inner diameter of the piston wrist pin bore, by insertion of the ring into a sleeve/tube. While still in the sleeve/tube, the wire lock ring is subsequently inserted, in its compressed state, into the pin bore where it is then uncompressed and released into the locking groove of the piston bore through the use of a plunger inserted in the sleeve/tube to push the lock ring past the end of the sleeve/tube.