tap sizes TAP Size Confusion

[RedJim]

A novice question, I'm looking for a replacement crown for my Zodiac Guardsman Automatic. I've measured the stem at 0.70mm. Done some research and according to the Esslinger website that's a TAP12. What's confusing me is that on some other websites and Ebay they say TAP12 is 1.2mm and TAP7 is 0.70mm. Its a threaded stem (couple of pics below). It's conused me and I want to make sure I order the right thing. Sorry for starter question just getting into this hobby which is both fascinating and absorbing (and addictive). The Stem comes to flush with the top of the case tube and I believe I'm looking for a watherproof crown. Can anyone help and advise? Thanks all.

[KOwatch]

As far as I've been able to determine:

There's two different systems of stem measurement, metric, and "Tap size" which is one of those traditional and strange bigger-is-smaller systems like wire gauges. Unfortunately, they also get used interchangeably, so sometimes a place that uses metric will express it as "Tap 9" when they mean 0.9mm, or both, like Cousins.  If they say "mm", I'd go with that. If they say "Tap" and don't also list the size in mm, it's hard to be sure. Esslinger uses the old up-is-down Tap system as they show in their Tap size table.

I tend to only buy from sellers that explicitly list in mm, or when I know that the stem they are selling on ebay is the correct diameter based on the movement number because they only come in one diameter.

Also, sometimes you just have to buy stuff to measure it and find out for sure. Sometimes you order a part and it's the wrong size. It took me awhile to figure this out. It's one of the learning costs I've accepted. The sellers don't always have or are willing/able to find out the measurement you need. I've tried to find out all sorts of measurements for crystals and pushers, and there's no real way of knowing without ordering it to find out. Apparently the old-time watchmakers just know this stuff,  which I guess you would after a lifetime of ordering different parts and establishing supplier relationships.

[JohnR725]

Like everything else in watch repair there's probably a tool or something for doing something. First link is your crown gauge. Then sizing of stems that you mentioned is found on the second link. 

 

https://www.cousinsuk.com/product/crown-gauge?code=G0693

https://blog.esslinger.com/stem-tap-mm-size-cross-reference-chart/

[MichaelG]

Agreed that a tool like the gauge in the link is best to measure stems and crowns but it is pricey. I chose instead to buy eight stem extenders in different sizes, about a dollar apiece. They come from Esslinger in individual bags marked both tap and mm. I try different sizes for the stem or crown I’m measuring until one fits.

I’ve never used a stem extender but I guess an added benefit would be always having one on hand.

Mike

[jdm]

40 minutes ago, MichaelG said:

Agreed that a tool like the gauge in the link is best to measure stems and crowns but it is pricey.

One can measure stems with a $10 digital caliper, and crowns with stems of a known size, as you said. Beside, normally mechanical watches are almost all 0.9mm.

[rodabod]

Jim, do you know what the base caliber is for the watch? You may be able to purchase a more common size of stem if it is indeed an uncommon 0.7mm thread. I think you may struggle to find a suitable crown if it is 0.7mm. Another option would be to use a thread adapter/extender to convert to a more common size. 

[JohnR725]

2 hours ago, jdm said:

One can measure stems with a $10 digital caliper, and crowns with stems of a known size, as you said. Beside, normally mechanical watches are almost all 0.9mm.

I sort of have one of these gauges? I hadn't really looked I knew they change the color but it looks like they change some of the sizes my gauge only goes down to 0.9. I purchased mine like a very long time ago didn't pay very much at least I don't remember it costing much and for the most part I don't think I ever use it at all.

[rodabod]

I use a crown / stem gauge regularly. It’s the only way to truly confirm a stem thread dimensions, as long as they are ISO/metric profile. Vernier calipers are ok for modern stems which are very likely metric as you only need to confirm the major diameter. But sometimes you need more detail such as when you have a stem with a unique thread like on earlier JLC, IWC and Longines watches. 

[RedJim]

Thanks for all your help everyone, much appreciated.

[Acidstain]

0.7mm is very odd for a mechanical movement. Double check your meaurements with a digital calipher.