Service: Omega Seamaster Cosmic with 601 Movement

 In Watchmaking

This Omega Seamaster Cosmic with a 601 movement was a tough call.

The dial, hands, and the front of the case are in good condition, but the back looks like it was used as a block plane.

During the inspection, it was immediately clear that it needed a service because the movement was filthy. I also noticed that the sweep second pinion friction spring needed to be replaced because it was severely distorted. The crystal also required to be replaced.

Based on the condition, the initial verdict was that this watch wasn’t worth the time and money. The owner would sell it as is.

However, it pains me to see a non-runner. Especially when it’s a watch like an Omega that would be a good runner with some love and one tiny new part. I decided to ask around for a used spring in good condition.

Luckily, I was able to find one, so here we go.


Remove all power from the mainspring as usual. If you don’t, you might damage the escapement or the pinion of one of the wheels when you remove the pallet fork or while you’re working on the wheel train.

Move the click away from the ratchet wheel and slowly let the crown slip between your thumb and index finger. Be careful not to let it snap because that can damage the mainspring.

Remove the balance and the pallet fork with the pallet cock and store them somewhere safe. I always store them in a tray underneath a dust cover. You can also use a balance tack.

Omega Seamaster Cosmic with Omega 601 movement
Balance and pallet fork removed

Lift the sweep second cock and the train wheel bridge to get access to the train wheel.

Omega Seamaster Cosmic with Omega 601 movement
The train wheel exposed

Remove the sweep second pinion, the 3rd wheel, the 4th wheel, and the escape wheel.

Be careful with the sweep second pinion friction spring! It’s this movement’s weak point.

If this spring is still in good condition, it’s a good idea to remove it and store it somewhere safe so you can’t damage it during the cleaning process.

This spring was badly bent in this particular movement, and that causes problems. The spring touched the shaft of the sweep second pinion, and that creates friction which results in a loss of amplitude and bad timekeeping.

The spring should lift the pinion away from the center wheel cock, but it should not press against the sweep second cock. In other words, the pinion should “float” between the sweep second cock and the center wheel cock, and that’s what the friction spring does.

If it’s severely bent, it’s tough to bend it back in exactly the right shape, so I ordered a replacement.

Remove the ratchet wheel and the crown wheel and lift the barrel bridge. Often, the crown wheel has a left-handed screw, but this one has a crown wheel boss with two small right-handed screws.

Remove the center wheel cock and lift the center wheel and finally the barrel.

Omega Seamaster Cosmic with Omega 601 movement
The top plate empty. Look at all that dirt. Where does it all come from?

Flip the movement around and start with the bottom plate.

Remove the motion works and the keyless works.

The shock protection system on the bottom plate is not an Incabloc like the block on the top plate. This system consists of a cover plate with a spring, a jewel hole, and a capstone. Completely dismantle it and place it with the other small and delicate parts in the cleaning machine.

Omega Seamaster Cosmic with Omega 601 movement
The bottom plate pre-cleaned with a peg wood and Rodico

Pre-clean the pivot holes and bearing jewels with a peg wood.

Clean all the parts in the watch cleaning machine.


First, put the shock protection system back together and place it in position on the bottom plate. Don’t forget to lubricate the capstone.

Flip the movement around and fit the barrel, the center wheel, and the center-wheel cock.

Install the barrel bridge and the (new) sweep second pinion friction spring.

Omega Seamaster Cosmic with Omega 601 movement
The barrel, center wheel with the center wheel cock, and the barrel bridge placed in position

Place the escape wheel, fourth wheel, third wheel, and the sweep second pinion in position.

Install the wheel train bridge and then the sweep second cock.

Install the crown wheel and the ratchet wheel. Don’t forget the washer for the crown wheel.

Omega Seamaster Cosmic with Omega 601 movement
The wheel train, the wheel train bridge, sweep second cock, ratchet wheel, and the crown wheel reinstalled

This is an excellent time to test if everything works and if the wheel train rotates freely without any friction. Install the cannon pinion before you do that so that the center wheel is secure.

Turn the movement around and fit the rest of the motion works and the keyless works.

Omega Seamaster Cosmic with Omega 601 movement
The bottom plate completely reassembled, apart from the hour wheel and the dial washer

Flip the movement to the top plate and reinstall the pallets. Wind the watch a couple of times to check the pallet fork. It should snap to the other side with the lightest touch.

The movement should come alive when you place the balance in position.

Now it’s time to clean and lubricate the top Incabloc block. Use a peg wood to clean the capstone before you clean it with One Dip or something similar. Lubricate the capstone and reassemble it.

Place the hour wheel and washer in position, fit the dial and continue with casing the movement.

I fitted a new crystal because the old one was damaged beyond repair.

Omega Seamaster Cosmic with Omega 601 movement

The crown with the female part of the stem and the male part of the stem inside the movement should be replaced as well because they’re badly corroded.

However, they still work fine for the moment, and this watch isn’t worth these new case parts in this condition.

Omega Seamaster Cosmic with Omega 601 movement

What do you think of this Omega Seamaster Cosmic? Would you add it to your collection in this condition? Let me know in the comments below.

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Showing 2 comments
  • Ewald Termaat

    Well, to answer the question…
    Yes. I bought it!
    The precious owner sent me the link to this website and I’ve just red the story about the repair of ‘my’ Cosmic.
    Very interesting!
    It will probably be shipped tomorrow, so I hope it will arrive this weekend!

    • Melvin Hollenberg

      That’s great! Good to know it’s found a good home. Enjoy wearing it.

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