This section will deal with how to upgrade the Crown on your 3646 Case. To do this I will drill the Case and add threads to accept a new Stem Tube and "Onion" Type 3646 Crown.
The Stem Tube I will use is 4mm and the thread pitch is 0.35mm
First start by drilling the case. The recommended size is 3.4mm but I am using a 3.6mm drill bit (slightly larger). This will make the threading process a little easier. Using a 3.6 drill bit is adequate, the threads may not be quite as deep but they are perfectly strong. The stem I am about to fit is not heavy duty so it does not require any real strength other than to hold itself in place. I will later Tap (thread) the 3646 Case using an M4 (4mm) Tap with a 0.35mm thread pitch.
Tapping the Case: Its important that the "thread pitch" you make in the case is the same as the thread pitch on the stem tube. The hole in the case is now 3.6mm and I will start the threading process using a M4 Tap with a 0.35mm Thread Pitch (easy to buy on the internet).
To make the threads I will use a set of Three Taps. Tap 1 is the Starter (a fine weak thread). Tap 2 is an intermediate thread (a little deeper) and Tap 3 is a Full Depth Thread.
- Use the "Starter" Tap 1 the make the first few "light threads". I prefer to make a few starter threads to "at least" half way into the hole in the case. Make sure you apply pressure to the Tap so that the threads dont slip, the Tap needs to constantly cut new threads. Remove the Tap from the case
- Next take Tap 2 (Intermediate Tap) This will re-cut the previous threads you have made with Tap 1 (starter tap). Once this Tap is half way through the Case there is usually enough strength to proceed with this Tap so that it goes right through the Case.
- If you look at Tap 2 the threads start light and continue to full deep threads. Therefore there is no need to finish the case off using Tap3 once Tap 2 has gone through the Case.
Now its time to try the Stem Tube and see how it fits.
The next stage is to Solder the Stem into the Case. The authentic Cases were soldered in the same way. This wasn't to make the Stem strong, it was instead to make it water-tight when diving under pressure.
Before we start to Solder, REMEMBER to remove the small rubber o'ring from inside the Stem Tube. If you dont it will melt when you start to solder.
Type of Flux & Solder: To do this I would strongly advise using the very best flux and solder (there is a kit available on the Vintage Watch Parts website). The correct flux makes a big difference between the work being fairly easy and difficult. Dont try doing this work with cheap flux, and always us "fine" solder. You do NOT want to get solder on the Stem Tube Threads.
First apply the Flux around the Stem Tube (inside of the Case). Next use a "Flame" (Gas Blowtorch - NOT electric) and heat the flux, the flux will go black, next, try and touch the solder to the case it should melt on contact. As soon as you have some solder on the case add a little flux to the solder, the solder should easily flow. In fact the solder should "Sweat" inside the case and into the threads, sometimes it can be visible on the outside of the Case (stem tube area) this means the solder has fully sweated through the case. Its not always possible to do this so dont worry if you cannot.
Tip: What does it mean if you add solder to the case and it doesn't flow but stays in a large ball or blob or molten solder. Often it means your Case is TOO HOT. Cool with flux and it should flow like water.
Next, run solder around the top edge of the Stem Tube to form a nice seal. Be careful not to get solder on the threads. This is why I say you must use fine solder and good flux.
Once Soldered you should clean the Case by polishing. Remember to put the rubber o'ring back inside the Stem Tube
Below is a comparison between Crowns. The Old Crown is larger and more common on modern Panerai Watches. The 3646 Onion Crown just like the Vintage Crown. Its not always important to have the correct 3646 Crown because during the late 1940's (and early 1950's) there was a parts shortage and it is very common to see authentic 3646 Watches with Rolex Crowns fitted.
One final upgrade - The owner of this watch wanted Lead O'rings fitted under the Bezel and to the Case Back. Lead o'rings were used as seals on the authentic watches but many didn't survive over the years or were lost.