Service: Zenith El Primero Power Reserve 03.0240.4021 / calibre 4021

We’ve done quite a few El Primeros here at the workshop, but never the calibre 4021. The 4021 is a modification of the calibre 400.
Instead of the hour recorder and date, the 4021 has a pretty nifty power reserve indication. On top of that, the movement is skeletonised around the balance and escapement, so that these components are visible through an opening in the dial.

The dial looks a bit Breguet-like, and there is of course no hour recorder as on the calibre 400.

This watch was sent in from Lithuania, just to make the whole thing a bit more interesting 😉

A good thing that the watch has a crystal case back – there are a lot of nice things to see in here!

With an amplitude of 205 degrees, the movement needs a service.

The case back is screwed into place with 8 screws, guaranteeing a tight seal.

There are a further 8 screws for the bezel ring.

A bit of dirt got into the case, but not past the gaskets.

The skeletonised movement exposes the escapement and balance through an opening in the dial. The lowest hand is for the power reserve indication, which we will explore in more detail later.

The dial with the opening.

A very different bottom plate to the standard calibre 400. The escapement is exactly in the same place, but opened up for visibility. Under the cover at the top right lurks the power reserve indication mechanism.

The top plate of the 400 and the 4021 are identical.

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

All chronograph elements are nicely visible with the chronograph bridge removed.

The usual abrasion on the intermediate winding wheel post – an issue Zenith hasn’t addressed properly since the conception of the movement, and the reason why I would go for a manual wind Zenith.

The cut-outs made into the plate are well done, e.g. they are leaving enough material not to destabilise the plate.

The crown wheel with its plastic ring, which avoids wear.

The gear train with autowinder intermediate wheels at 11 o’clock.

This is the power reserve indication mechanism. The little steel pinion at the top is driven by the barrel, and drives the outer planetary wheel (bit wheel at the bottom), which drives the power reserve indicator wheel through a clutch mechanism. When winding the watch manually or automatically, the barrel arbor drives the inner planetary wheel, turning the power reserve indicator wheel in the other direction.

Here, the planetary gear open. The steel coloured wheel sits in the middle, and is driven by the barrel arbor.

And here is the power reserve indicator wheel with a slot. The slot holds a pin from above, limiting the movement of the power reserve indicator hand from empty to full. Once it’s on full, the little clutch will slip, and the indicator will stay on full and not go any further.

A super construction, and there is nothing about it that I don’t like.

The setting lever with its spring. 

The cleaned parts ready for reassembly.

No lack of bits and bobs 😉

As usual, I start off with the gear train.

Then the crown wheel assembly goes into the bridge.

In order to be able to wind the movement for testing, the keyless works need to be assembled as well.

The wheel bridge is in, and the movement is beating. With a complex movement like this one, I make sure that everything is just so before proceeding.

This includes adjusting the movement, and testing in all positions.

Now that that’s done, I proceed with the power reserve indication.

The inner planetary gears still have to go in.

The power reserve indication is in, and I test it before proceeding further.

Now I put the chronograph back together. That’s the easy bit, as I have done quite a few 400s in my time.

The movement is back together, and I test everything again.

Now the dial and hands go on, and I fully wind the watch before setting the power reserve indicator hand slightly right of the “+” marker.

New gaskets for the case.

The case is back together, and I waterproof test. Everything is well.

I quite like the Breguet-style dial and the hands – all going together very well. The last step is 3 days on the autowinder, and a final power reserve test.

13 thoughts on “Service: Zenith El Primero Power Reserve 03.0240.4021 / calibre 4021

  1. As I own exactly that timepiece 4021 I was fascinated of the wonderful fotos.
    Additionally it looses time and I was remembered to have a maintenance.
    Thank you for that and the interesting foto story. See you.

  2. Interesting to hear about the abrasion. If this i present in all Zenith 36 000-movements, could that be the reason that Rolex de-tuned their El Primeros down to 28 800?
    Also, do you know if this abrasion is present in other Hi-Beats, especially those made by Seiko?

  3. very well done. I quite like the dial even though I’m not a fan of skeletonised dials.
    Is the abrasion caused by the beat rate of the movement?

    • The main reason is the gear ratio of the rotor to the mainspring I think. As the 36,000 bph needs a pretty big mainspring, the autowinder has to provide a lot of power, too. But a higher gear ratio might have worked better, with less abrasion.

  4. Just two questions:
    1) There is a “3 handed seconds” that is never removed from the 4-th wheel post. Any special reason? Why not to treat it as a normal hand and remove it?

    2) As mentioned the “usual abrasion on the intermediate winding wheel post” is evident! Looking at the other job carried out previously with similar movements its always there! Is this transverse to all El Primero Zenith movements like 3019?

    Best regards,

  5. I like that and a nice servicing job. I have the El Primero in an old Ebel, Don Johnson style but maybe one day I’ll get a Zenith one too.

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