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THROUGH A GLASS DARKLY - Insights Into Crystal Variances and Construction

Authored by Swedefreak

[size=1em]In this wacky word of WISdom I seem to learn something new every day. This is a good thing, of course, as in my "real life" learning something new each day is one of my rules.

After corresponding with a few fellows in the past few months and looking very closely at my stock of genuine NOS crystals for the 6105-8110/8119, I have discovered there are no absolutes. The 320W10GN00 was used from the late '60s into the '80s in no less than 64 seperate Seiko models.

Until recently I was under the impression this number was for a single version of this well-used crystal. It turns out there are at least four variants, all carrying the part number 320W10GN00:

1. The oldest (?-nasty part packet) has a flat top and inside dome. It measures 3.85mm high with frosted sidewall and polished bevel.

2. Second variant is double domed measuring 3.83mm +/- 0.02mm high with frost and polish as above.

3. Third is double domed but is only 3.63mm +/- 0.02mm high with frost and polish.

4. This one in a newer style (late '80s) Seiko packet is double domed and 3.63mm high but has a polished sidewall and bevel.

I have several of numbers 2 and 3 but only one each of 1 and 4 so my evidence is semi-complete. I wish Seiko was more involved with we who preserve thier vintage product. It would be great to know the true whens and whys of these changes.

I can only guess based on three and a half decades of working this stuff but here goes---

Flat top crystals are terribly reflective. Double domes are far less so which is why this makes sense for a dive watch. Also, the double dome is much more structurally sound than flat or flat/domed.

The crisp edge of this crystal is a chip magnet. In the early '70s I remember seeing fairly new watches coming in for new crystals, within weeks in some instances, due to chips and nicks on the top edge. My assumption is Seiko got tired of the complaints by'74 or '75 and dropped the height by 0.20mm. This doesn't sound like much but it's enough to get that edge closer to the bezel of a sport watch or the rotating ring of a dive watch.

As a manufacturer of replacement crystals I can tell you it is more expensive for each additonal step in the process. This one reason the sidewall of most TMG crystals is not polished. The other aftermarket guys should realize this in addition to making an incorrect product. But the true reason for not polishing the sidewall is accuacy of diameter. The cutting process is finite. You want 32.00mm, that's what you get from the cutting machine. Polishing glass of any kind is not so finite and material removal is not so exact which leads me to believe crystals with polished edges may not be quite the right size. And when we're dealing with the close tolerances of any watch case, size does matter.

I hope this hasn't put anyone to sleep at their keyboard. It just seemed like a good idea to share some observations. Thanks to the fellows who sparked this diatribe. You know who you are.
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