Folks, we're discussing a Grand Seiko here - of course this was within COSC standards when new. In fact, Seiko's own precision standards were (and are) even tighter than COSC standards. So after the service it ought to be possible to have it run within COSC standards.
In general, vintage watches usually run surprisingly accurate - after all, that's what they were once made for. In an age when people didn't have radio time on cell phones to check, a well running wristwatch was what was required. In fact, most of my vintages run within +/- 30 seconds per day - those that don't are either completely worn out and kept just for sentimental reasons, or in need for a service.
And, just to put "one minute per day" in perspective: this is a deviation of 0.07%. I'm not aware of many technical systems working with smaller tolerances ;-)
BTW: This is an interesting read on the 1960's Grand Seikos and their obsession with beating the Swiss on accuracy: Vintage watch experience: Seiko
And why I ended up wanting to own that piece of history. It's amazing to think how they got -3/+5 (and sometimes better on their VFA models) back then.
I'll see how it does after service, but even if they can't get it within those specs again, I'm happy to see it in my collection.
Again - why shouldn't this be possible? Your watch was designed to run with +3/-1 sec per day and unless its bowels are extremely worn out, there is no reason why these values can't be attained after a decent service. I'd like to point to my Franken-Omega with the famous 564 Chronometer calibre ...
... or the Mido Oceanstar Datometer with its purpose-designed chronometer-movement AS 1920:
Agree with all previous about your Seiko, it would be nice to see how close to perfection they can get it. The widely received wisdom, quoted on endless sites, is that a vintage watch is thought to be accurate + - 2 minutes a day. Many will disagree, and there are huge variables between models and so on, but I'm simply reiterating 'what people say.' But as an example, I have a JLC P478 that's running around 5s fast a day, not bad for a 70 year-old watch, I think.
The problem with Seiko watches is that they were so good they often were used untill they just wore out so it's always a gamble with them. Grand Seiko were the top of the line though and I suspect most were taken care of.
Every week, usually on Monday morning, I choose 5 watches from my collection of vintage Elgins, wind them fully, and set them using a quartz watch as a reference. I line 'em up, and wear one a day through the work week, but I wind ALL of them each day. With rare exceptions, they stay within 2 minutes for the entire week. Mostly, they stay within 1 minute. My "wake up and put on a watch" watch is wound every morning, and stays within a minute every two weeks. And these were all serviced by an amateur.
When they were new, they all kept time within a few seconds a day for the lower grades, and better than that for the higher grades. Sure, a vintage watch can be so worn, and far enough out of adjustment that it varies 2 or more minutes a day, and it can be tough for an amateur to get it consistent again, but I think seeing 2 minutes a day variance as acceptable is really misleading about how accurate they can be.
Vintage accuracy will always vary, becuase the main determination is how it was used and serviced during it's life. Mid-grade watches may not have even left the factories doing better then +/-15s/d in some positions; and nobody really cared. Rolex "Chronometer" rated watches may have LEFT the factories meeting COSC specs, but it doesn't take much for dust and bits of metal to start building up inside the case; even a "Oyster" isn't 100% sealed. Oils break down, especially in hot environments, and in worst cases, the combination of dust/metal/old oils creates an abrasive paste around the pivots. This cases the pivots to become scratched, which increases the friction they experience. And that kind of damage is difficult to "fix" without replacing parts.
This is a particular problem with 70's and 80's era watches, because the advent of cheap, disposable quartz watches made it increasingly difficult to justify the cost of servicing a mechanical, so there's a greater chance of people just letting the watches run till they stopped. And that's even more of a problem for many Seiko's, because they'd tend to run longer under poor conditions, so when they finally stop, they're stopped for good. Not sure how true that was for the higher end products though.
I would say it depends on how often I use the certain vintage watch. The watches I wear just occassionally(few times /year)-Im happy with +-2min/day, talking about mid-range products. Vintages that I wear more often or daily, my tolerance is from +-15 to +-30sec depending of course on the watch, I expect more from handwind watches that are famous for their precision. With some vintage models I pushed my watchmaker to regulate it until it reaches factory specifics, thankfully he's pacient but what to do, I'm a lifelong customer
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