The replacement of the drag knob is a little difficult because the knob is a tight press fit.
To remove the knob firstly remove the spindle screw, D washer and then the spool.
With a Philips head screw driver remove the drag pawl and washer, the drag cam will now slide up the spindle.
To enable the knob to be removed you now have to drill the centre out of the knob, a drill up to 8mm (5/16″) will do.
Once the knob is weakened a pair of multigrips should assist in removing the knob by collapsing the drilled out lever into the hole.
To insert the new knob, place the lever section through from the inside of the back, support the back allowing the knob to come through.
Using a wooden dowel and hammer give the inside of the knob a light tap to seat in the recess.
Replace the drag cam on the spindle, make sure it spins freely.
Replace the drag pawl with the drag pin (under the pawl) placed in the slot of the drag cam.
Replace small washer and tighten self tapping screw, then back screw off half a turn.
Make sure pawl and cam can move freely, adjust screw if required.
Place drag in off position 0.
Replace spool, D washer and spindle screw.