Adjusting an Unadjusted Gruen N510 Wristwatch to 5 Positions, Part 1

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Some watch companies get too much love from collectors, but a few get overlooked. I think Gruen is one of the underappreciated ones. Gruen watches have their small following of devoted collectors, but they don’t get the attention that Hamilton and the big Swiss brands get.

I recently picked up this Gruen “Precision” wristwatch: a model N510, part of the big family of 510 movements, from sometime in the 1950s, I think.

The watch has contrary markings. On the dial it says “Precision,” obviously a claim that this watch is pretty accurate. (It also implies that their other watches are imprecise, but we’ll let that go.) On the movement, however, it says “UNADJUSTED.”

All good watches were always adjusted. During this period, import duties were higher for adjusted movements, so many watches were marked “unadjusted” to skirt the taxman. Good watch companies nevertheless adjusted the movements to at least a couple positions.

A wristwatch like this was probably adjusted to the main 3 wristwatch positions, which we described in an early post: dial up (DU), dial down (DD), and pendant down (PD). We don’t know the standards Gruen used. At that time, a difference between DU and PD of 30 seconds was seen as pretty good—that would be marginal today.

Like many Gruens, this watch looked like drawer fodder. It probably spent the last 40 years in a junk drawer, getting nicked and scuffed. The crystal is a lost cause and has a big ring of grime along the edge.

The upper case is gold filled. It’s heavily worn around the lugs, and there isn’t much you can do about that short of sending it off for an expensive restoration. But the stainless back should clean up nicely.

The winding stem rusted and needs some quality time with a fiberglass scratch brush. I might need to replace it.

The watch movement itself was clean and in good shape. After the usual ultrasonic scrubbing, I started to reassemble it.

Finding mainsprings for Gruen watches can drive you batty. Watches with huge followings among collectors are easy to identify and find parts for. There isn’t nearly as much info out there on Gruen as there is for vintage Hamilton, Longines, JLC, or (pauses to look dramatically into the middle distance) Rolex wristwatches. I found this mainspring chart, but it wasn’t much help for my N510.

So it was off to the calipers and the Internet. The original mainspring’s height is 1.30 mm, and the barrel’s diameter is around 10.5 mm. It took 3 mainsprings to find a good one. A .11 thick mainspring was too weak—the dial-up (DU) amplitude didn’t get past 260 degrees. And a .13 thick mainspring was too strong—the watch knocked like a door-to-door knife salesman.

Here’s what I settled on (a GR3310: 1.30 x .12 x 300 x 9). A .115 thick spring would be fine, but we’ll take the extra amplitude when we can get it.

The watch ran well, but the hairspring wasn’t flat. At first I feared that it had become distorted during the cleaning cycle, but the problem was weirder and easier to fix. The movable hairspring stud carrier was slightly bent. Notice that the stud end (the part with the stud screw) tilts up a bit.

This metal is surprisingly soft, and bending it back to flat corrected the hairspring.

Adjusting the Little Fellow

After demagnetizing and letting the watch run for a couple days to settle in, it was time to see what kind of adjusting could be done with this little watch.

The omens for precision adjusting were not good. The regulator is a simple “nudge me” regulator, so fine control of the rate is going to be tricky.

And the balance screws are the cheap, flat-faced variety. We saw those on the FEF pocket watch I recently serviced.

In most cases, you can use a balance screw holder to grip the side of the screw and then thread it on and off the wheel. But in some cases, the screw holes weren’t tapped for repeated use, so a screw, once removed, might not thread on easily and seat snugly against the wheel.

Your best bet is to set the watch to run slow and then poise the wheel by removing weight. In this case, I opted to file flat against the screw face using escapement files. I wouldn’t do this on an Illinois Bunn Special, but these Gruen screws are already flat so filing the face won’t change how they look.

As for the good omens? Well…the amplitude was great (over 310 hanging, over 260 lying) and the timing machine traces were clean, so there’s that. Otherwise, we’re hoping to squeeze accuracy out of a little watch that wasn’t originally adjusted for it.

The initial rates were not bad at all.

The dial up and dial down positions are very close, and that’s the first step in dynamic poising. The vertical rates aren’t too bad: the biggest gap is 18 seconds (PU and PD).

The biggest flaw, in my view, is the gap between DU and PD. These are the 2 main wristwatch positions, so it would be nice to have them be closer than 26 seconds apart. We’ll aim for 5 positions. (The sixth position for wristwatches is PR, which is circled.)

As we’ve described in other posts, we’ll poise the balance wheel by finding its heavy spot. We find that spot by running it at low amplitude and finding the fastest rate.

The watch ran fastest in the diagonal position between PL and PD.

That means that the heavy spot is directly below the balance staff when the watch is in this position.

So, we could increase poise by removing weight from the heavy spot or by adding weight to the light spot directly opposite it. The watch is running slow, so we’ll remove weight from the heavy spot by lightly filing a screw at the heavy spot.

Remember the dynamic poising mantra: if the watch is running fast dial-up, add weight to the light spot; if it is slow, remove weight from the heavy spot. (And prefer adding weight when feasible.)

After around 3 or 4 rounds—filing, testing the rates at low amplitude, and filing a bit more—we ended up with these rates.

This is pretty good. It isn’t perfect—you’ll notice that the 4 hanging rates are all a little slower than the 2 horizontal rates—but it is a very good result for a watch like this. The 3 main positions—DU, DD, and PD—are all pretty close, and the biggest error is thrown into the 6th position.

Wrapping Up the Innards

This humble and neglected Gruen turned out well. The rates are all closer together, and it is ticking away with great amplitude.

The next task—do something with the rank and crusty case. See you in Part 2!