1963 M 100-4 Automatic

My most recent post was an uncatalogued model and today's post is too.  I have it on good authority that it's an M 100-4 but don't have a catalog image to share.

I'm guessing that it's from 1963 because in 1964 there was an M 100-3 and an M 100-5.  The dash (-) means it's the 3rd and 5th model priced at $100.  If the -4 was to occur, it would have to be before the -5 and this the -5 is a 1964 watch, then the -4 must be earlier.  How's that for logic?

The M 100-4 came in a yellow 10K RGP case with a stainless back.  Tucked inside is a 17 jewel 689A movement, made by ETA.

I've had an M 100-4 for a long time, several years to be exact.  However, I never serviced and I rarely wear it.  Since I don't have one on the blog yet, I thought I'd overhaul it and post it for posterity.

I've seen a number of M 100-4's in the wild.  It's a little easy to spot because it has numbers only at 12 and 6 and relatively long markings for the minutes (or seconds, I suppose).

The stainless steel back has fooled many people into thinking this is a two-piece case.  But it's not - it opens through the crystal.

I like to use Bergeon tools whenever practical.  They are rather expensive but the quality is excellent. The 4266 crystal lifter is my go-to choice for opening one-piece cases.  The tool compresses the crystal around the perimeter, squeezing it down to a slightly smaller diameter and allows the crystal to come out of the case.  Putting it back in is a breeze too.

The 689A is a typical ETA movement.  A couple of minor changes to the oscillating weight carrier is the main difference between the 689A and the 689.  This case incorporates a movement ring to secure it inside the case back.

This style of ETA movement is interesting because it uses a special cannon pinion that is inserted inside a toothed wheel.  The hour wheel was removed to show the cannon pinion.  The cannon pinion can slip inside the wheel and allow the time to be set without jarring the gear train.  Unlike a conventional movement, the cannon pinion is driven by the third wheel pinion and not the center wheel.  In fact, the center wheel isn't even in the center, it's off to the side in the lower left in the picture.

Everything is cleaned and dried before being reassembled.  The trickiest part of this movement is to get the four-wheeled train bridge properly seated.  Sometimes it drops right in place and other times it can take 20 frustratingly-long minutes.

Without the rotor in place, the movement looks like a garden variety manual winding movement.  Now comes the movement of truth - putting the balance assembly back on.  If everything is lined up correctly, the watch will start running.

Success - the watch is running so it's off to the timer to see how well it's ticking.  It's a little fast but I'll leave it like this for now.  It's not uncommon for it to slow a little after it runs a bit.

From there it's just a matter of putting the rotor back on as well as the dial and hands.  With everything assembled, it goes back into the case and I can push the crown back onto the female side of the two-piece stem.  Then the crystal goes back on and... voila!  It's finished.