Servicing the Seagull 3620 (Unitas 6498) – Disassembly

In this tutorial, I will show you how to disassemble the Seagull 3620 movement. This movement is a clone of the Unitas (ETA) 6498 and thus the same process can be used to disassemble both movements. There is also a Seagull 3600 movement that is almost identical to the 3620, however, the second-hand subdial is located at 9 o’clock, rather than at 6 o’clock which is the case with the 3620. These movements feature very similar architecture, however, the arrangement of the bridges are different to accommodate for the different locations of the subdials. The Seagull 3600 is also a clone of a Unitas movement, the Unitas 6497. Therefore these instructions can be used to dismantle both the 3620, 3600 as well as the Unitas 6498 and the Unitas 6497.

In order to disassemble this movement, a basic selection of watchmaking tools are required. Please see my watchmaker’s toolkit post for more details on these tools. A link to this post can be found below.

Watchmaker’s Toolkit

To disassemble this movement we will begin by unwinding the mainspring. This is done by placing the movement into a movement holder (dial side down) and locating the click. The crown should then be rotated a fraction so that the click is sat on a tooth of the ratchet wheel, a small screwdriver can the be used to retract the click so that it clears the path of the ratchet wheel. It is important to maintain a grip on the crown at all times to avoid damaging the mainspring. The crown should then be allowed to slowly rotate between your fingertips until it comes to a stop. Alternatively, the watch can be left for a couple of days for the mainspring to run down by itself.

The movement should then be turned dial side up so that the hour wheel can be removed using a pair of tweezers.

The cannon pinion should be removed next using a cannon pinion tool.

The movement should be turned once again so that the dial side is facing down. The screw that secures the balance bridge should then be unscrewed. Screw heads can be easily marred by ill-fitting screwdrivers, therefore it is important to use a screwdriver that correctly fits the slot in the screw head to avoid such damage.

After this screw has been removed the balance bridge and balance wheel can be carefully lifted off of the movement. To avoid damaging the delicate hairspring, the only part that should be handled is the balance bridge, allow the balance wheel to dangle below the balance bridge and take care when placing the balance bridge into the parts tray to prevent kinking the hairspring, or snapping the balance staff.

The next step is to unscrew the two small screws that secure the pallet bridge to the mainplate.

The pallet bridge can then be lifted off of the mainplate. Although the bridge is secured with screws you will that the bridge will remain firmly attached to the mainplate, even once these screws are removed. This is because locator pins are used to align the bridge and often these are a snug fit. You may find it helpful to use a small screwdriver to help gently pry the bridge off of the locator pins. This applies to all of the bridges in this movement and often there will be a small slot on the edge of the bridge for this very purpose. Take extreme caution when removing all of the bridges to avoid bending or breaking the pivots. I recommend dedicating an old (or cheap) screwdriver to for this task as you may find that prying may damage the screwdriver.

The pallet fork can then be removed. As with many parts of the movement, the pallet fork pivots on an incredibly fine arbor, so treat all parts with great care to prevent these fragile parts from becoming bent or broken.

The crown wheel screw can now be removed – note to remove this screw it should be turned clockwise, as it has a reverse thread. The crown wheel can then be removed.

There is a small crown wheel ring (bushing) attached to the crown wheel pivot that also needs to be removed.

Next, the ratchet wheel screw needs to be unscrewed, and then the ratchet wheel can be removed from the mainspring arbor.

The click screw should then be removed.

The click can now be removed, as well as the click spring that is located beneath the click.

The three screws that hold the barrel bridge to the mainplate can now be unscrewed. The barrel bridge can then be carefully lifted off of the mainplate. Again you may require the aid of a screwdriver to pry the barrel bridge from the mainplate.

Next, the two screws that secure the train wheel bridge need to be unscrewed.

The train wheel bridge can then be removed.

The centre wheel is then removed from its jewel.

Now the main spring barrel can be removed.

The third wheel can then be removed.

And next, the second wheel.

The escape wheel is removed next.

Next, the movement should be turned so that the dial side faces upwards and the screw that secures the setting lever jumper should be removed.

Now the setting lever jumper can be removed.

The next step is to carefully remove the yoke spring. I recommend placing a finger on top of the spring whilst removing it to prevent the small spring from being catapulted across the room!

Next, the yoke can be removed.

The setting wheel can now be lifted off of its pivot.

Next, the intermediate setting wheel can be removed.

The minute wheel is removed next.

Now the movement should be turned once again so that the dial side faces downwards. The setting lever screw can now be removed.

After you remove this screw, you will find that the setting lever has fallen onto the work surface and should be picked up to avoid it being misplaced.

The crown and stem should then be pulled outwards and the sliding pinion can be removed from the channel it slides in.

Finally, the winding stem and winding pinion can be removed from the main plate.

The movement is now fully disassembled, with the exception of the mainspring barrel assembly and the balance bridge. You may wish to further disassemble the mainspring barrel for cleaning purposes. Although the balance bridge can be disassembled further, it is not usually required.

The parts can then be cleaned using your preferred method, I will be posting instructions as to how I clean watch parts shortly.

The movement can then be reassembled and lubricated, I will also be publishing a step by step tutorial on this process shortly.

I hope you have found this post to be informative and interesting and if you have any questions please do get in touch. If you would like to be notified when I post further tutorials including how to clean and reassemble this movement, please subscribe to my blog.