Caseback integrated with movement and crystal does not lock into bezel

[LittleWatchShop]

This is an odd configuration.  The movement fits into the case back and the crystal friction fits on the case back.  Then the whole thing mounts into the bezel (for lack of a better term).

When I took it apart, there was enough grime and rust to keep it all together, but after cleanup, the module (case back + movement + crystal) just falls out of the bezel.

Not sure what to do.  It cannot go as is.

The first image shows the module alongside the bezel.

I do recall some stuff fell out from around the module.  Maybe it was glue from the previous repair.  GS Hypo or something.

[Geotex]

That appears to be the style used for many Benrus watches (what you call a module I call a capsule). They sold a special press with unique dies for assembly and disassembly, but manual pressure is often sufficient, especially since nobody is expecting them to be waterproof anymore.

The thickness and fit of the crystal is an important part of the geometry needed to get a tight engagement between all the parts. I wonder if yours was a not-quite-thick enough replacement crystal that required some additional glue when it was put together because the OD didn't allow a snug fit? 

[LittleWatchShop]

41 minutes ago, Geotex said:

I wonder if yours was a not-quite-thick enough replacement crystal

Could very well be.  I have a lot of GS crystals here, but it will be a process of poke and hope because, both inside and outside diameters are important as well as a vertical profile for the sides.  Frustrating.

 

[Marc]

This system was also used on a lot of Roamer watches of a certain era.

As @Geotex points out the sizing of the crystal is important. Its fit to the case back/capsule needs to be snug, and its outer diameter needs to be a friction fit into the bezel. That way when everything is pressed together a good water resistant seal is achieved, keeping everything secure as the bezel compresses the crystal tightly onto the back. There shouldn't be any need for glue unless the right size crystal is no longer available. If that is the case then get a crystal that fits the case back as well as possible but has too great an outer diameter and carefully sand it down to a nice tight fit into the bezel.

[spectre6000]

Like Marc says. I had one like this a while back. Actually, I still do because I'm having the opposite problem and haven't gotten around to dealing with it. You need a crystal that's a bit thicker around the waist. In my case, after digging for a silly amount of time, I found a crystal that was ever so slightly too chubby, and I need to take it down a bit. I just haven't figured out a clean way to do so that won't likely make it look off, or be asymmetrical, etc. You're well tooled up, so it shouldn't be too difficult. I ended up calling whichever crystal manufacturer I found that seemed like they had something close, because their catalog was missing some critical dimension. The lovely woman on the phone dug out a few part numbers that looked like they might work sans the missing dimension, and measured them over the phone. Unfortunately, that was quite a while ago, and I couldn't tell you who that was if I had to.

[LittleWatchShop]

Problem solved in a rather unusual way.

Since the old crystal is in good shape and fit the movement perfectly, I did this.

Took a piece of aluminum foil and covered the crystal and over the module (capsule).

Then I pressed that into the bezel.

Using a surgeons scalpel I cut away the foil.  It is a very tight fit and you cannot see the foil.

The foil, incidentally, is 20 microns thick.  That is all it took.

My method is easily reversible (i.e., I can press the movement out without harm).

[spectre6000]

Brainthought: Any concern with galvanic corrosion? No worries on the plastic, unsure about the case. (Digs up galvanic series chart) Aluminum is super low on the series, so you're probably fine. Still... Maybe something to consider. 

[LittleWatchShop]

On 10/28/2022 at 8:28 AM, spectre6000 said:

Brainthought: Any concern with galvanic corrosion? No worries on the plastic, unsure about the case. (Digs up galvanic series chart) Aluminum is super low on the series, so you're probably fine. Still... Maybe something to consider. 

No, I really did not think about it.  Perhaps I should have.

I delivered the watch today, so it is off to a new life.

The owner was very happy...it was his grandfather's.  I do not take money for my work, but I do get paid. 

[Geotex]

I'll post this in case others come across this topic in the future. Since Benrus handily provides the patent number right there on their cases, it's easy to find the concept diagram on file. It depicts a rectangular case but the fit is the same for rounds.  The drawing shows that the crystal edge terminates in a squared-off section with parallel sides of a defined inner and outer diameter to precisely fill the gap left between the case and the bezel.

. 

Edited December 28, 2022 by Geotex