Servicing a Seiko 6138 Bullhead Automatic Chronograph - Observations on Vertical Clutch and Hour Register

You are looking at a fully disassembled vintage 1970’s Seiko 6138 bullhead chronograph on my bench being serviced. This is a complicated column wheel automatic chronograph with a vertical clutch design. It includes a day/date register, 30 minute register, and 12 hour register.  The first picture shows the chronograph disassembled and the second shows various aspects I will discuss in this blog. 

Relative to finish and visual appeal, the movement is as-expected focused on function and not beauty. Two significant aspects of the movement design are its vertical clutch and chronograph hour register. Let’s walk through the 4 quadrant picture clockwise. In the first picture, the arrow shows the vertical clutch which drives the chronograph and runs constantly. When the chronograph is engaged, friction is reduced in the clutch and the chronograph runs. Most other chronographs, with a few exceptions, use horizontally sliding clutches which either engage, or do not, the chronograph seconds wheel based on chrono pusher activation. Note that a few high-end chronographs also use a vertical clutch, so this is not necessarily a cost reduction approach. Each design has its pro’s and con’s but the fact that vertical clutches are pre-lubricated and effectively “sealed” at the factory, cannot be fully cleaned as a result, and not serviceable makes me less pre-disposed to them though they can continue to function for decades. The next picture shows the mainspring barrel with integrated chronograph hour register driving wheel (red arrow.) The friction tension spring for this driving wheel is shown with the blue arrow. The third picture shows the hour register wheel with its tension spring used to provide tension for the hour register brake. This spring is adjustable vis an eccentric screw to control whether the hour register is “lazy” and incorrectly runs without chrono activation (see my other post on lazy hour). The final picture shows the day date dial side.

Seiko 6138 chronograph fully disassembled (Seiko bullhead 6138-0040)

Seiko 6138 chronograph servicing observations - vertical clutch and chronograph hour register