Threads stripped

[genazzano]

The chrome bezel or retainer ring on the rf window of a Leica III was missing when I bought the camera a few years ago. I finally got a replacement from DAG recently but found that the piece would not screw in straight. A close examination of the threads in the camera body showed that they had been xthreaded at some time and will not accept the male threads of the screwing bezel.

There is about 1mm of thread and I need to clean and retap or replace the entire upper body.

Does anyone know the if it can be retapped or should I begin looking for a new upper body?

David

[Stephen]

David,

The thread will be a metric fine pitch thread, and the pitch can be determined from the screw in part. It could be 1mm, .5mm or .75mm pitch, most likely .5mm pitch. Examine with a high power lens and good steel ruler.( A camera lens will provide a good lens) or use a Jeweller's Loupe lens.

The diameter can be checked from the thread in by measuring the overall size, I have not got an example to hand from a Leica, but say 7mm, 8mm, or 10mm. It is likely to be a dead number in millimeters.

The appropriate tap can then be sourced, "Tracys Tools" in the UK can supply most odd sizes, but there is a potential serious problem.

The size may not be a "Preferred Size", and the tap not commonly made.

The answer is to have a thread restorer made, it would need a lathe machinist to make a plug thread of the right size in steel, and relieve the thread with cross cuts, and it is then eased into the damaged thread, by hand, using a guide bush.

The threaded restoring bush will reform brass threads easily, it both cuts and "forms" the softer metal.

However there are other ways, and a tap of a small diameter, but the same pitch, can be used to "chase" out the thread, it is eased sideways into the damaged thread, and the cutting edge with the tooth pattern will "scape" away the damage.

It needs skill, and patience, to do this, and it must be done with magnification like Jewellers glass's on, to really see what is going on.

A single point hand scraper or gravure cutter can also be used to scape the thread back clean, it would need the end ground with a right angled crank tip.

For both the restoring thread tap, or a restoring plug with cross cuts, they must be entered with a "plug guide", a blank of aluminium or brass bored to take the tap, or restorer plug, with clearance over the teeth.

This is clamped over the hole to ensure the tap or restoring plug enters at exactly right angles, and removes any chance of the cross threading happening again during restoration.

With great care, a hand tap could be used without the guide, but it needs skill and judgement to maintain the thread without further cross threading occurring.

It should not need a new casting or parts, at worst the offending thread could be drilled out, and a new threaded sleeve loctited into the part, with a threaded centre to take the chromed bezel.

I am not suggesting this for a proper standard Leica repair, but the damaged thread could be filed away and the whole bezel popped in with a spot of loctite anyway! The use of Loctite is recommended for this, as the part will be removable later on.

Hope these notes help,
Stephen.

[genazzano]

Thanks Stephen. As usual, your help is indispensable. David

[Stephen]

Thinking about it, does not the Leica III have a threaded bush that sticks out from the casing, and the outer chromed bezel screws onto the thread, the bezel having the internal thread? Exactly the same comments apply if it does, except you will need a thread die rather than a tap, if the cross threaded part is a male thread on the body side.
Russian versions screw in direct to a threaded hole. I am not sure what Reid did or other clones.

Stephen.

[genazzano]

The iiif does as do others made after 1945 or so. This one was upgraded to iif some time after 1953 but Leitz kept the older iiia body. The female threads in the body are pretty messed up after closer examination. This camera is a superb user but not a collector so I may keep watch for a replacement upper housing.