When the central guide sleeve of the piston assembly in an older 149 cracks or breaks, the guide lug no longer runs in the sleeve's longitudinal slot. The mechanism then twists on itself instead of driving the piston along its axis, and the pen no longer draws ink. Repairs normally start at the piston knob, which sits on the piston on a left-hand thread and usually comes free with a forked tool and gentle warming. If it has been glued as well, warming, an overnight bath in penetrating oil and ultrasound often achieve nothing, and sawing it off destroys a part that is next to impossible to obtain.
In that case the answer is to reinforce the sleeve from the outside. A brass ring with a stepped bore is slid over the free end of the broken sleeve; a collar about 0.5 millimetres high bears against the sleeve axially, and the ring is bonded in place with epoxy resin. So that the ring will pass over the seat of the cork seal, that seat is first reduced in diameter and then brought back to its original dimension by the stepped bore. The width of the ring is dictated by the piston in its retracted position; around 0.3 millimetres of axial play stops the piston jamming at the end of its stroke. That restores both the anti-rotation function and the full piston stroke without sacrificing the glued-on filling knob.