Zenith ‘El Primero’ 410 CHRONO Moonphase full servicing
Photos by Finn Marjoram
Let’s review a nice example of the Zenith El Primero chronograph Cal. 410 – We’ll breakdown the full servicing process and some watchmaker pros and cons.
We disassemble the whole movement to service.
El Primero servicing cons – Too many parts!
| Too may parts | There are twice as many components compared to any other typical chronograph. |
| Different screws length | One bridge, three screws. All have the same size screw heads but are different lengths. Why? It should be illegal to make screws with identical heads but longer or shorter threads! The Watchmaker needs to be extremely careful storing screws while the whole movement is disassembled. Almost every screw used in this movement has different length and there is an abundance of them. |
| Day / Date contour alignment | The Day disc instant change system is overcomplicated – it has 30+ parts and must be aligned in quite a counter-intuitive way while assembling. It’s really good experience for any watch manufacturer to study this system but to never (never!) design anything like this. |
| Comparing to Valjoux 7751 | While working on the El Primero, you can’t avoid comparing it to ETA factory ‘workhorse’ – The Valjoux 7751 caliber has all the same functions, including the Moonphase full calendar. However, it has significantly less components, everything is thought out and clear. Very similar movements but differ greatly in serviceability. |
El Primero servicing pros – It’s beautiful!
All things considered this watch is pretty. Essential parts are really precise and well finished and the component overcomplications looks really sophisticated on one side. It’s a pure joy to see this movement from the inside and work on it, to see how it’s built and how it’s functioning. You can’t get this experience with normal use.
Due to the complexity of the movement, when you successfully assemble it after cleaning, you feel like you’ve climbed a mountain. And the best part is that when you assemble everything correctly, it really works well alongside it’s full set of overcomplicated functions!
Notice the Moonphase disc core is riveted to the module plate.
Have you noticed the broken date jumper spring above? How could a wire spring snap like that? Weird.
Finally it’s assembled, oiled, adjusted, tested and ready to be encased for the final 2-week cyclic testing.
Ordering and prices at Horological Underground
Our typical servicing prices (2025):
El Primero Moonphase+Calendar calibre servicing £630, 12-month warranty
El Primero Date only calibre servicing £540, 12-month warranty
We take servicing/repair orders from the UK and abroad, please see our page dedicated to Watch Repairs in Horological Underground Bristol.
