Wristwatch restoration, servicing and repair

Oris Star ChronOris (Oris Cal. 725)…

When introduced in 1970, the ChronOris was the first Oris watch with a chronograph function. I was very surprised to find this one locally, and it’s the only one I have ever seen with an orange dial.

(Click pictures to enlarge)

Over the years this watch was made with a number of different dial designs and two different shaped cases; a cushion shaped case and the one like mine with an external Tachymetre bezel.

You may have noticed that unlike a conventional chronograph, the ChronOris only has one pusher, successive pushes of which start, stop and reset the sweep hand back to zero.

You may have noticed too that the watch has no minute counter, but don’t worry you don’t have to keep track of the minutes yourself as the inner orange/white bezel can be rotated using the crown at 3 o’clock. By lining the pointer up with the current position of the minute hand, you can time events of up to 60 minutes. The knurled crown at 4 o’clock takes care of winding the watch and setting the time and date.

Inside this watch is the Oris cal. 725; a 17 jewel, 18,000bph, manually wound calibre with a chronograph mechanism which was designed and developed in conjunction with Dubois Dépraz, the renowned chronograph specialists.

Here are a few pictures of mechanism in action, the first one shows the mechanism when the pusher has been pressed for the first time, starting the chronograph.

The power for the mechanism comes from the transfer wheel which is attached to the fourth wheel in the going train, so turns continuously (along with the intermediate wheel) whether the chronograph is engaged or not.

The key to the whole mechanism is the column wheel whose position determines which levers or wheels are affecting the chronograph centre wheel. You can see that the lever attached to the intermediate wheel is currently between two pillars on the column wheel so the intermediate wheel is now in contact with the chronograph centre wheel. As the sweep second hand is attached to the axle of the chronograph centre wheel, on the dial side of the movement, the sweep second hand would now be moving.

A second press of the pusher stops the chronograph.

The pusher moves the column wheel around, disconnecting the intermediate wheel from the chronograph centre wheel, the brake lever falls in between the pillars of the column wheel and contacts the chronograph centre wheel, holding it in it’s current position.

A third press of the pusher resets the sweep hand back to zero.

The pusher moves the column wheel around again, lifting both the intermediate wheel and brake away from the chronograph centre wheel. This time the hammer falls between the pillars of the column wheel and moves across to contact the chronograph heart, rotating it back to zero from the stopped position.

With the movement back together I cleaned the hands and dial as best I could, cleaned the case and polished the crystal. It is still possible to order new crowns and pushers from Oris, but as the Tachymetre bezel is no longer available there didn’t seem much point. The watch shows it’s age here and there but I don’t mind, I’m going to enjoy it just as it is.

I couldn’t find much information about the original ChronOris, but I did read that only 27,000 examples were made, which seems like a small amount for such an iconic Oris model.

To commemorate the original, Oris released a new version of the ChronOris in 2005, based this time on the Valjoux 7750 calibre adding a second pusher, a 30 minute counter and automatic winding functions while still retaining the retro styling of the original.

If anyone has any more details about the original ChronOris it would be great to hear from you.

Rich.


One Response to “Oris Star ChronOris (Oris Cal. 725)…”

  1. Peter Says:

    Very interesting story! Has been the reason why I now bought one of these. With a turquoise-colored dial and orange-white bezel.
    Unfortunately I have no additional informations. Thank you for this nice presentation.

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