Thursday, February 13, 2014

7751 Chronograph Base Movement

Today in class we did some practice for some of the tasks likely to come up on the CW21 test.  These include a staff replacement and a barrel bushing manufacture and install.  Once I had finished with those I started a reassembly of a 7751 which I was assigned yesterday.  It's the same as a 7750, but has a more complicated assembly on the dial side.  Below you will find step by step (more or less) assembly of the base movement.  The chronograph components will follow shortly.

These are the oils I will be using for this movement, from left to right (9415)(9010)(molycote dx grease)(D5)
Here is the oiling and assembly chart for the winding and setting mechanism

Grease is used on the sliding metal-on-on metal surfaces.  I wick the D5 heavy oil into the parts which rotate on a pivot point.  Here from left to right you have the: time setting gear, yoke spring, rocking bar, yoke, sliding and winding pinions and the setting lever

All of the setting components are held down by this combined setting lever jumper/plate which also holds the intermediate setting wheel.  Notice the white molycote on the jumper teeth to the lower right

Now that winding and setting are taken care of, time to flip over to the movement side and get the movement side components installed!
The going-train assembly instructions

Moving counter-clockwise from the large silver barrel at 12 o'clock you have: the great whee, third wheel, second wheel, escape wheel.  The thin silver metal bar in the middle is called the hack.  This stops the watch when you move the watch into time-setting position
After installing the barrel bridge plate which holds all of the train wheels down it's time to install the pallet fork.  Here it is with its bridge installed, you can see how the jewel pallets interact with the teeth of the silver escape wheel.

Now the rest of the winding components, here I have applied grease to the area where the crown wheel rides.  It's important to have this area generously lubricated due to the large amount of friction latent in the design.

Lubricating the underside of the plate that holds down the crown wheel

Crown wheel installed, I also wicked some oil down underneath the return to zero operating lever above the wheel.

Now all of the wheel pivots must be oiled.  Everywhere you see a red circle must be oiled (in this case with either D5 of 9010)  with the exception of the pallet jewel which does not receive oil.  The barrel arbor also needs oil.  This must be done on both sides of the movement
Here is the ratchet wheel installed, minus its screw.  This is the wheel that connects directly to barrel allowing winding but also making sure it does not unwind freely due to its interaction with the click.  The click in the thin metal wire you see engaging the ratchet wheel at 4 o'clock.

Time to oil the balance.  Here I have removed the in-setting and cap jewel from the block.  The shock spring and balance pivot are visible.

These parts are extremely small and very easy to lose if you are not careful.  The watchmaker must put a tiny drop of oil (in this case 9010) on the cap of the assembly then sandwich these parts back together.  Once installed in the watch it lowers friction on the balance pivots tremendously.

Here I am setting up the regulating pins for proper timekeeping, it's hard to see but the hairspring must be perfectly centered between the regulating pins before you close then to be just shy of touching the spring.  This guarantees the best time performance of the watch in differnt orientations.

Time to make sure the base movement is keeping good time before moving on with the chronograph assembly

The delta is good for this movement at 6.2, amplitude is okay at 262 between all positions and rate is slightly fast, but we save final regulation for the completed watch.


  1. This comment has been removed by the author.

  2. Hey Andrew, very cool blog!! One question on average how much the amplitude drops with the chronograph on? I recently acquired an Omega with the 1151 caliber (7751). Chrono OFF my amplitude average is 262, and with the chrono ON average is 224, is this acceptable?

    1. It was the goal in school that the drop in amplitude should be less than 30 degrees for a newly serviced movement. I haven't worked on any ETA product since I left school however, so I don't have any real world experience to know whether or not any more than 30dgs is normal for a watch that has been running for a while. I suggest giving an Omega service center or jeweler a call, they'll know for sure.