As we know 100 barrels, made of celluloid, get brittle with time, especially internal thread part. Of course, replacement plastic barrel is (was?) available, I chose to save the original celluloid parts as possible as I can.

I restored broken internal thread (Fig.1) by using TAMIYA epoxy putty (Quick Type). as when fresh, epoxy putty is soft enough to mold with fingers. With the knob unit (Fig.2) inserted into the barrel (Fig.3), epoxy putty applied on the barrel and external thread of the knob unit, and compressed it carefully to form internal thread. Before that, knob unit was greased to prevent it from sticking together. After epoxy putty turned solid enough, carefully removed the knob unit (Fig.4). Finally, carefully shaved down unnecessary epoxy putty with the craft knife (fig.5) to shape the internal thread part of the barrel. You can see internal thread made of epoxy putty (Fig.6, 7). When solid, epoxy putty is firm enough to engage with the knob unit (Fig.8, 9). Major crack can be mended without ink leakage(Fig.7 left). Epoxy putty may come off the barrel while shaping (Fig.6, 7). Then, it can be glued with Loctite® Epoxy adhesive.

Caution about the procedure.

I finished after much trial and error and almost ruined one barrel. The process needs experience, time, carefulness, and patience. If available, I would recommend replacement barrel.