Vintage and antique crystal-sizing frustration.

[MrRoundel]

I seem to be wasting a lot of money on crystals lately that just don't fit right. One of the recent challenges is an odd case, the Benrus/GP Seahawk style where the outer-case ring is the bezel and band for the case. You have to fit the crystal over the top of the case back before lowering the band/lug-piece over it. With crystals being sized by outside diameters, and with inner diameters meaning a lot with this type of case, I'm on my third try for sizing. With that watch, I'm as much into crystals as the watch cost me! OK, well maybe that one will work out with the next crystal. I'm awaiting a package from Cousins that has the new crystal in it. Fingers crossed.

So then I buy an antique pocket watch from just past the turn of the century. These are what I cut my teeth on, so to speak.  When I get the watch, I find that the hunter crystal that's on it is very undersized, and was somehow being held on by the back of the bezel. Slick move by somebody, right? I take my watch to a friend who is an expert on pocket watches. He tries some crystals he had in the bezel, but only had one of the size and didn't want to give it up. He said it was a size 18 10/16. Since my experience tells me that crystal-fitting only sounds like an exact science, I decide to measure the bezel at least 5 times to be sure I get the right crystal. It measured between 41.26 and 41.30mm. After doing the math, using 2.254mm per ligne, I figure that I'd need an 18 5/16''' crystal. Now that's a bit smaller than the one my friend had that was marked 18 10/16, right? Like .7mm!   I find an 18 5/16''' on Ebay, and order it.  The crystal comes in today's mail in one piece, but is a bit too large for the bezel. This is a glass VTF geneve glass for the hunter case, so I won't be able to use my crystal press on it. I measure this crystal, complete with its factory VTF size-tag on it of 18 5/16, and it measures between 41.55 and 41.6mm. This should also be known as an 18 7/16 lignes. Are these crystals famous for being mismarked? I'm measuring with a digital caliper that has always been reliable. I suppose this is why watchmakers keep a lot of crystals on hand, as they seem to be like Forrest Gump's box of chocolates, where you never know what you get. Bloody hell. Next...

[khunter]

I've found that the sizes listed on the old glass crystals is more of a guide than anything. I've measured several that we all marked the same, but still varied in diameter by a tenth or two from what was stamped. You CAN resize a glass crystal, but it is painstaking and not always successful. I made a wooden bobbin on my lathe, about 1" in diameter, then slightly beveled one face, and coated it with a thin layer of silicone rubber sealant. Mounted on a 1/4" dowel, I chuck it in my lathe and use the tailstock with a loose fitting wooden "pusher" to hold the crystal to the bobbin. Run the lathe slowly and use hand pressure on the tailstock while centering the crystal on the bobbin, I use the wooden handle of a burnisher to apply pressure to the edge of the crystal until it runs true. Then I tighten the tailstock in place. Using various grits of carbide emery cloth, I gently reduce the diameter a little at a time. You MUST polish the sanded edge using the finest grit cloth you can find, at least 2000, or when you try to install the crystal it WILL chip! Heat the bezel up! This will slightly expand the ring and allow the crystal to go in, and when it cools it will shrink back down and hold the crystal tightly.

[MrRoundel]

Thanks, khunter. That sounds like it's more effort than I care to expend, as well as being well beyond my abilities. I do see that it would work, providing one took their time and used the right abrasives, etc.

The seller found another crystal that is marked 18 4/16, and measures 41.27mm. The actual measurement of mine is closer to ~41.35mm, so I'll have to use a bit of glue. I'll see how it goes. One thing for sure, the 18 5/16 ligne wasn't going to do it without the type of work you described above. I tried heating the bezel, as well as cooling the crystal, but it wasn't enough to make up for the .15-.20mm size differential. Thanks again. Cheers.

 

[khunter]

One other thing i have found is that the old crystals are not perfectly round, so I measure in several different places and get the average. Sometimes just changing the orientation in the bezel will be the trick to getting it to fit. All that being said, I've had much better luck with GS acrylic crystals, especially in hunting cases, because the glass is so thin to begin with. Good luck! 

[transporter]

I’m finding it near impossible to de sypher the sternkruz sizes, especially for odd shapes, I’ve a rotary waiting for a crystal but it’s oval in shape and I cannot find a size close to it in their catologue, it might just be me but crystals and crystal gaskets just don’t get on sometimes

[Tudor]

I think GS has a better selection than the Germans. You’ll need an old catalogue from when the watch was current to get the GS number but I e seen a lot of unique shapes over the years. 

Note I avoid odd shape cases that need crystals for this reason!

[rappeleur]

Odd shapes are truly a pain in the ... neck. Especially because the original ones got yellow and opaque through the decades. Is it allowed to post ebay links? I found one seller (jonsmv) with various sizes/shapes respectively tailor made crystals! Here’s the link to the eBay auction.

 

(I also had deviances with old crystals up to about 0.2 mm in comparison to the description on the sleeve, not limited to Sternkreuz).

Edited April 18, 2018 by rappeleur