Need for Replacement of Shutter Curtains
Spring Drum for determining shutter speeds.
This part is what we call spring drum of Leica M type camera. It is an important part for giving tension to the shutter curtains, and determining shutter speeds.
Shutter curtains are set up on this part. When you release the shutter, spring inside of this part pulls the curtains and shutter curtins start to move. Most cameras have two spring drums for the first curtain and the second curtaion.

This is that the spring drum has been taken apart. You can see the inner spring, whose grease has been deteriorated. The spring drum become like this after 10 years or more since production or overhaul (OH), there is an individual difference though. Once the spring drum has been like this, it could not keep the shutter speed accurate. Only adjusting shutter speed is nothing more than makeshift repair, the shutter speed will go wrong again soon.
In this case, cleaning and lubrication are necessary.

Cleaning and lubrication
This is after cleaning and lubrication. In this condition, we set the spring drum into the camera, and we adjust the shutter speed. Of course, the shutter speed can be accurate by not only cleaning and lubrication of the spring drum, but also overhaul of the whole camera.

Without lubrication of the spring drum, you cannot keep the shutter speed accurate even if you adjust it. If the spring drum is in need of lubrication like above, the shutter curtains are damaged themselves. (The shutter curtains should be damaged even if there are no defects clearly visible, for example the curtains have a hole or they are ragged). The shutter curtains are made by gluing cloth and rubber together, and the rubber gets constricted and becomes hardened with time. Due to the shutter curtains are glued, you should not reuse shutter curtains removed once as much as possible. To conclude, when you do overhaul of old cameras and cameras which overhaul has not been done for a long time (inclusing replacement of shutter curtains and lubrication), you cannot say that you did overhaul without above work.