Service: Zenith El Primero calibre 405

IMG_9568The Zenith 405 is a true flyback. Just to avoid confusion, let me explain what that means… Lots of people think just because the hands on their chronograph “fly back” when they re-set it, it’s a flyback. It’s not. The difference between a normal chronograph and a flyback is that you can press the reset button whilst the chronograph is running, and it will instantly re-set the chronograph hands to 0, and continue to run from there. Apparently a feature that was useful to WW2 pilots, and, even though probably not of much use to you and me, something sought after!IMG_9570


A first look at the 405. It’s a 400 with a couple of modifications, so most parts from the 400 fit.IMG_9571

That’s not too bad.IMG_9576

The minute recorder hand is slipping and doesn’t reset to 0 properly, and that’s because the tube of the hand isn’t attached properly to the hand itself any more. I solder the tube to the hand to resolve the problem.IMG_9577

The bottom plate with the hour recorder.IMG_9579

I start off with the chronograph layer on the top plate.IMG_9583

This Zenith chrono has very little damage and abrasion, which is rare. Most of the time, it’s a battlefield in here, with lots of abrasion and worn down parts. Not in this case.IMG_9592

I just have to pull off the driving wheel for the chronograph, and then I can take off the wheel bridge.IMG_9595

Ready to take off the bridge.


The gear train with the winding gears.IMG_9607

Now it’s time for the bottom plate. The winding / setting / date quick-set mechanism is quite complex.IMG_9610

The unlocking date spring is still the old type, and I will replace that with the sturdier new construction.IMG_9616

All ready for the cleaning machine. You can only buy a complete barrel for £100, so I will re-use the mainspring.IMG_9617

Whilst the movement is in the cleaning machine, the case will go into the ultrasonic cleaner.IMG_9638

The old mainspring is still in very good shape, as you can see.IMG_9640

Now I put the balance jewels back in.IMG_9641

The gear train is in place, so I can put the bridge back.IMG_9642

Time for some lubrication. In this photo, I’m lubricating the escape wheel with Moebius 9010.IMG_9643

I put the winding gears in so that I can wind the mainspring.IMG_9645

That’s the beauty of Zenith chronograph movements. 10 beats per second, and an arrow-straight beat rate.IMG_9646

The base movement beating again.IMG_9647

Now I start on the chronograph layer.IMG_9649

It’s 5 o’clock, and time to go home. The movement goes into a dust-proof container.IMG_9650

It’s the next day, and I complete the bottom plate. You can see the new-style unlocking date spring in place.IMG_9652

Ready for the dial and hands.IMG_9653

The hour recorder looks rather sorry, with paint chipped off, so I will do something about it.IMG_9654

Firstly, I take off the old paint with a tiny drop of paint stripper.IMG_9656

And then I re-paint using highly diluted white model paint, applying it with a fine oiler.IMG_9657

When putting the chronograph second hand on, I notice that it slips as well, so I solder it to its tube, too.IMG_9658

Finally, the dial and hands are back on (bar the hour recorder hand, which is still drying).IMG_9661

Time to case the movement, and to put the oscillating weight back on.IMG_9663All back to its former glory, with the newly painted hour recorder hand.


19 thoughts on “Service: Zenith El Primero calibre 405

  1. Hi Christian,
    Do you think that the Zenith Rainbow Flyback is a good everyday watch (Is the EP 405 robust as it was intended) ?


  2. Hi.

    I am writing from Spain.

    I have a Zenith Rainbow flyback like the one you show in this thread.

    I wanted to ask you a question.

    How can I disassemble the bezel and its insert ring in this watch?

    The reason I want to do it is this:

    The bezel of this watch has small recesses that make it possible to grip and turn
    in either direction. Not all recesses are the same. Every five minutes the recesses are larger and approximately semicircular.

    Logically, and for aesthetics, these major recesses should coincide with minutes 0, 5, 10, 15, etc, 50, 55. As you show in your photos.
    In mine they are slightly deviated and coincide with minute 2, 7, 12, …, 52.

    Is there any way to extract the bezel as it can be done in other watches, for example, in Seiko’s and Rolex GMT’s?

    Is it possible and safe to do so just being an “aficionado”? I do not want to spoil anything.

    Thanks and regards.


  3. Pingback: Rainbow question

      • Hello again.

        Thanks for your reply.

        I bought the watch at the beginning of the year from an english dealer “the vintage watch shop” and it came to me that way.

        In the city where I live there are no watchmakers with enough experience in mechanical watches -and less in Zentih-; and the Zenith service is about 700 kms from my home.

        The watch, except for this small detail, is in very good condition and works perfectly, so I think that send it to Zenith for this small detail is an exaggeration.

        For that reason, and assuming perhaps wrongly, that the procedure to do it not differ too much from what I’ve seen in some Seiko’s and Rolex’s, is why I got in touch with you.

        Obviously if the procedure differs a lot, it includes a lot of risk and I had to have very specific tools then I will not try it.



        • Hello again, Christian.

          A last chance.

          Any clue or idea to disassemble the bezel – and its insert (Is this stuck or just pressed?) – without producing an irreparable disaster?

          Thanks in advanced.



  4. Pingback: <--------WRUW Thursday May 1, 2014-------->

  5. Hi Christian,

    I’m delighted you have been able to resolve the issue with the chrono hands on my watch. The previous person mustn’t have noticed what exactly was wrong as it seemed OK when I got it back but the minute hand drifted again, I kind of gave up on it after that ! At least the rest of the movement was OK

    Hopefully I’ll have it back in time to test at my Friday night football match.

    Incidentally I have a Heuer Stopwatch (Dembo Bristol also on the dial) which has the flyback feature.The start/stop is a slider, the crown resets.

  6. Hmm – I love a Rainbow Zenith 😉

    Why was it so clean in there compared to its contemporaries… maybe just not been used much?

    Interesting looking regulator on that movement – how does it work?

  7. Nice! I have the 400 in a two tone Ebel and I like how fast the date changes. There are slowed down clips of it on YouTube.

    Mine was recently serviced by Ebel and it came back changing over at 4am so I sent it back and now it’s at 11.40pm. Is it hard to set the change point?

  8. Hi Christian what a nice El Primero and in such good condition, unlike most of the others you have serviced which have large amounts of debris in them. I am interested how you solderd the hands without the heat traveling through and burning the paint. Many thanks again for a great blog regards Mark.

    • Yes, this one is in great condition, and a flyback on top!
      I’ll share my little secret here now …
      Put the hand on a flat surface, and put a tiny amount of flux around the tube. Then just touch the tube with the soldering iron (one of those small electronic jobs) until the flux dilutes.
      Then cut off a tiny tiny tiny tiny piece of electronic solder (I use a sharp small knife), and cut it in half, and in half again, until you can’t cut it any more.
      Put it right into the 90 degree spot between the tube and the hand, and then heat it with the soldering iron until it starts to spread. I have the hand on a metal surface to take the heat off. As soon as the solder spreads, take the iron off, and you’re done!

  9. Hi Christian:

    How much do you thin the paint for painting the hands? Do you then “drag” it over the hand with an oiler, letting surface tension do the work of distributing it evenly? Is more than one coat needed? I’ve refinished a few hands in my watch modding and fixing exploits, and find it’s very hit or miss.

    • Hi Stefan,

      I thin down until I’m close to water, and, as you said, I let surface tension do the work. I’m guessing here, but I’d say I add 1/4 – 1/3 of thinner to the model paint I use. You got to be quick, and spread the diluted paint evenly over the hand. A single coat is fine (if it covers, if not, apply a second one, but I rarely find that necessary).
      It’s a bit like applying luminous compound. It’s all in the viscosity of the paint (or compound), and, once you got that right, you will get it right every time.

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