The mainspring powers the train of the watch which is assembled later and, in this calibre, it also directly powers the chronograph hour counter so the barrel is a little more complicated.  This is the barrel arbor and it may be seen that the bearing surfaces are in good condition.

UK watchmaker showing service of Omega Speedmaster - microscope review of barrel arbor

The teeth on the outside of the barrel are used to drive the watch train as the barrel turns when the watch runs.  The shape and condition is important for consistent running as the barrel only turns once every six or seven hours so the meshing teeth very slow climb up and down these teeth.   These are all fine.

UK watchmaker showing service of Omega Speedmaster - barrel driving teeth

The mainspring is fitted to the barrel with the arbor in the centre.

UK watchmaker showing service of Omega Speedmaster - barrel assembly

In a typical watch, the barrel is now closed with the barrel lid but here, for the chronograph function to record the elapsed hours, this small gear fits on top of the barrel lid and drives the hour recorder wheel and hand.  This is only needed for this function.

UK watchmaker showing service of Omega Speedmaster - microscope review of barrel driving wheel

This gear is held to the top of the barrel lid by a friction spring attached with two screws.  The friction is critical so that when the chronograph function is disengaged and the hour recorder held by the brake, this gear will not turn as the barrel turns. If it does, and the hour recorder hand slowly turns then it is suffering “chrono creep”.  This is usually caused by lack of cleanliness and incorrect oiling in this area often due to these parts not being separated during the service but can also be caused by  incorrect tension spring adjustment.

UK watchmaker showing service of Omega Speedmaster - hour recorder tension spring

The barrel lid is fitted to close the barrel.

UK watchmaker showing service of Omega Speedmaster - barrel assembly

The assembly continues with the keyless works, that part of the watch which allows for winding and time adjustment.