Stress cracks in the thread of the barrel are a common form of damage on models such as the 139. Welding the cracks chemically usually fails to achieve the aim: the seam remains clearly visible and rarely stays tight for long. Replacing the complete barrel insert, the obvious alternative, has the drawback that the characteristic long dark lines in the ink window are lost.
A proven alternative is a fitted thread sleeve. The cracked section of thread is cut away and a collar about 3.2 millimetres long and reduced in diameter is turned on the remaining barrel. A precisely made thread sleeve is fitted over this collar and bonded with epoxy resin under axial pressure, so that a joint is produced which will take load. On a pen with a long ink window the vertical lines are preserved over their full length, and the external joint line is barely noticeable.
On pens with a short ink window the new threaded part is made to overlap by exactly the length of the window itself. The external seam then ends where the barrel turns black and opaque, and is practically invisible. The work calls for a lathe and experience with historic materials, and therefore belongs in a specialist workshop.