Help Me Order a Crown

[SAK335]

I tried to order a crown for a couple of watches that have the same specs, and what I got was so far off what I wanted, that I'm going to ask for help this time.

I have two watches, one with a Cattin C66 movement and one with an EB8800 movement.  They both have the same stem thread and the same crown tube dimensions so I can order two of the same crown.  I spent 40 quid on a fancy Bergeon crown gauge, but it's got thread sizes in mm and not tap sizes.  I don't know how to convert tap sizes to mm...

So the dimensions I want are:

Crown: roughly 5.8mm in diameter and roughly 3.5mm tall (I measured these from another watch I have and thought it would look nice on these also)

Crown Tube: OD: 2mm+ ID: 1.6mm Length: 2.6mm

Stem thread: 1.2mm

These are both dive watches, so I though that the double o-ring type would be best, and I need it in chrome.  I found a couple on Cousins site that are within .5mm on the diameter and near what I want on the rest of the dimensions, but they only seem to come in Tap 9 and higher.  I found a chart here: https://blog.esslinger.com/stem-tap-mm-size-cross-reference-chart/  and it suggests that my 1.2mm measurement means tap 7, but I read several other places that suggest that almost all dive watches of this vintage have Tap 9.  This is why I'm asking...

I appreciate any feedback and suggestions.  I usually order from Cousins, and if Tap 9 is the right answer, then I think this one is the best fit:  9 x Ø5.50 x 2.50mm x (H3.4mm) SS, Swiss

[SAK335]

Also, I just realized that perhaps the Bergeon gauge is perhaps measuring Threads per mm and maybe that's why the Tap conversion chart messed me up last time?

[watchweasol]

Have a look at Esslingers web site  and look up windingstem tap size chart,   then do the same for Cousins uk. For some reason cousins use a different tap per mm than all the others  so cross reference the mm size with the tap size. unfortunately the chart I have will not down load to the site.       cheers

 

[SAK335]

Cousins has this in the description:

So by that perhaps I should be ordering tap 12 (12 = 1.2mm)?  It seems more logical than tap 7 but strange that they don't follow the same protocols as everyone else.

[Marc]

It is somewhat counter intuitive but stem diameters and tap sizes don't work that way, it just happens that Tap 9 coincides with 0.9mm which makes it look like it does.

Have a look here for a Tap size to diameter conversion chart.

[jdm]

1 hour ago, Marc said:

It is somewhat counter intuitive but stem diameters and tap sizes don't work that way, it just happens that Tap 9 coincides with 0.9mm which makes it look like it does.

Have a look here for a Tap size to diameter conversion chart.

That has been discussed before and is just messed up. The Esslinger chart you linked has tap 9 being 1mm, who made that chart? is that authoritative? Where do "Tap units" come from? I can live without the answer, I believe one must only use diameter in mm, all the rest is confusing, inconsistent, and can lead to error.

[Marc]

31 minutes ago, jdm said:

The Esslinger chart you linked has tap 9 being 1mm

I hadn't actually noticed that, I just linked to the first conversion chart that came up in a Google search however I was always under the impression that Tap 9 and 0.9mm was the point where the two systems crossed over. It looks like either I have had it wrong all along or Esslinger has got it wrong, ther cousins listing suggests the latter.

But I agree with @jdm in that the simplest and most reliable system would be just to use the metric system.

Further Googlefoo suggests that the concencus is with Esslinger....

Edited February 12, 2022 by Marc
Additional info

[nickelsilver]

The Esslinger chart is correct. The weirdness of the sizing comes from the old Martin screwplate norm, where thread diameter gets smaller as the number increases. Somehow in the past this got stuck to stem and crown threads and stayed stuck.

[SAK335]

OK, so I *still* think Cousins must approach this differently.  From a different style of crown, on their site:

As you can see, they seem to be associating Tap Size with diameter.  This seems to be why the last one I ordered (which was Tap 7 because I was using the Esslinger chart) was far too small for the stem that I have to fit this in.  Has anybody here ordered from Cousins that can confirm this?  It seems very strange to me to go against the norm.

[Marc]

1 hour ago, SAK335 said:

Has anybody here ordered from Cousins

I have always bought crowns from Cousins and have never had a problem, but then I have always ignored the tap size number and ordered in metric based on the dimension that I have measured from the stem.

Measure the stem and then order to fit, but use the same unit of measure for both measuring and ordering, that way you can't go wrong.

[jdm]

3 hours ago, SAK335 said:

It seems very strange to me to go against the norm.

Master nickelsilver has explained above what the good old chart is and from where it comes from. Now in 2022 we can use millimeters instead, the most common, almost universal, stem size is 0.9mm, if you need that just order that, it's written in the description and on the assortment boxes too. If Cousins ships you something that is not as described (very rarely happens) report that and you will be refunded.

 

[SAK335]

OK, now I'm 100% convinced that Cousins is using the metric value as Tap value.  I had ordered a crown for these watches previously using the Esslinger chart, and since 1.2mm = Tap 7 in that chart, I ordered a Tap 7 crown.  I just found that crown and put it on my new Bergeon crown gauge, and it screws perfectly onto the 0.7mm post.

Here's the line item from the invoice:

By that logic I'm ordering a Tap 12 and let's keep our fingers crossed.