the wear plate 40 beneath the pinch plate and adjacent the bottom of the thread confining passageway 43 in the wear plate the retaining element 90 includes a body portion 91 which may be secured flush against the wear plate by the lower fastening 39 of the wear plate and by rivets 92 to the projection 42 bent at right angles to the body portion is a thread constraining arm 93 defining with the body portion a thread accommodating slot 94 preferrably the arm 93 is formed with an inturned thread detaining finger 95 tapered so as to readily admit thread into the slot 94 and to deter egress of the thread therefrom the free extremity 96 of the arm 93 projects into a recess 97 formed in the head end cover 60 or in a member carried by the head end cover so as to prevent thread from inadvertently being passed over the arm 93 on the first upstroke of the take up arm after thread introduction to the system as shown in fig4 the take up arm in drawing the thread into position between the pinch plate and wear plate will also draw the thread past the thread detaining finger 95 and into the thread slot 94 on each succeeding downstroke of the take up arm as shown in.