Montblanc keeps a monopoly on spare parts: caps, cap tops and other components are released neither to dealers nor to customers, and repairs are carried out by the manufacturer alone. Trying to obtain a spare through a specialist shop or an independent workshop is therefore hopeless from the outset. With discontinued lines such as the Rubber Line there is the further problem that stocks of spares run out a few years after a series ends — at which point even the service department can no longer help.
Caps from fakes are no way out; as a rule they match neither in thread nor in dimensions. The only practical course is to buy a donor pen of the same line on the second-hand market — a fountain pen or rollerball in the same finish — and take over its cap. Among collectors of older writing instruments, stripping donor pens in this way is common practice and often the only means of making a pen complete again. Economically it is defensible: a cap repair by the manufacturer would have cost well over 70 euros (as at 2017), so a used donor pen comes to much the same sort of money.