[Leica] M rangefinder adjustment, topless or not?

[John Collier <jbcollier@home.com>]

First of all I think that both Walt and Jim are completely correct. The most
common rangefinder adjustments are:

- - the rangefinder distance calibration, comprising infinity and close focus
adjustments, and

- - the rangefinder vertical alignment.

All these adjustments can be done without the top plate coming off.

The vertical alignment is adjusted through the "red dot" on cameras equipped
with it, otherwise through the cover screw located directly above the lens
mount. In the early cameras, it is a simple adjusting screw but on the later
cameras (M4-2 and up) it is an eccentric cam that requires a special tool.

The rangefinder distance calibration is done by the eccentric pivot of the
rangefinder arm and the eccentric mounting screw of the rangefinder arm
roller. The roller position determines the starting point of the rangefinder
arm travel (infinity) and the eccentric pivot adjusts the ratio of
rangefinder movement to lens cam movement by varying the length of the arm.
A shorter arm gives you a larger movement of the rangefinder pivot for a
specific amount of lens cam movement; and, a longer arm gives you less
movement of the rangefinder pivot for a specific amount of lens cam
movement. Simple! There are also rangefinder travel stops but we hardly have
to go into that now.

Now that is what Leica will sometimes do at a Leica clinic and would be
considered a minor adjustment. It would take a technician about half an hour
to perform. There are however other rangefinder adjustments which require
the top plate to come off to perform.

All three brightline frame sets have separate adjustments so that they are
in the correct position at infinity and close focus. The mechanism that
selects between each mask set when a lens is mounted is of course adjustable
as well. I could go on, but all this stuff is beyond most people's ability
(certainly mine) and useful only for repair technicians.

So who is right? Both are. For most regularly serviced cameras only minor
adjustments are required and you do not need to take off the top plate
unless there is a problem with the brightline mechanism. On an older camera
with suspect or no maintenance records, it is better to take off the top
plate and check, clean and  lubricate everything, adjusting where necessary.

Cheers,

John Collier