From an answer given on TZ-UK about this very subject.

http://forum.tz-uk.com/showthread.p...44319-Where-is-WR-failure-most-likely-to-occur&p=2538585&viewfull=1#post2538585

Reproduced below

Yes, and no...
This type was favored by Seiko in the 6105 and 6309 series diver's watches. These do not offer any degree of water protection above the standard non screw down crown.

This is the non-screw down crown design favored by Seiko, only one gasket and it is internal to the case tube.

This is the standard non-screw down crown favored by the rest of the watch world, the gasket is external to the case tube. Note that in all cases of the simple single gasket crown, the gasket still seals the watch even in the extended position. This is true of all designs. So, leaving the crown out, theoretically, does not alter the water resistance.

These next two are screw down crowns that have a compression gasket above the case tube. The difference is which gasket is the primary seal. With the external thread (below) the compression gasket is the primary and the tube gasket is the secondary. The Vostok design illustrated above by Huertecilla, is the externally threaded design but the secondary gasket is omitted.

With the internally threaded screw down crown (below), the primary seal is the tube gasket.

The Rolex Triple-lock crown is a combination of the internal and external screw down crown

There are advantages to each of the various designs, so choosing one over is not just on a whim.... The external thread design is the easiest to manufacture, but the secondary seal will always wear out first, so in the event of primary seal failure there can be total failure, however this ia a very rare thing to happen*. This is the reason Vostok opted to discard the secondary seal, it really doesn't do much, unless you are in the habit of leaving the crown unscrewed...
The advantage of the internal thread is the primary seal is larger and can be designed to last longer than a smaller gasket, also it is the one that will wear out first, leaving the secondary to hold the line until the next service, the disadvantage is the threads are smaller and more easily stripped.
There is one other WR seal design that is rarely used, the cap. The cap in this case actually pushes the crown down slightly and this compresses the gasket as well as spreads it into firm contact with the case tube and crown. The advantage here is during turning of the crown, there is little compression on the gasket so wear is reduced.

 

In this case I would say that the plastic lid itself acts as the gasket. As you screw down the cap, the plastic deforms under pressure, closing off gaps and making a good seal.
In the case of a watch, the gasket deforms to fill in the gaps between the metal surfaces not only from expansion and contraction but also the small imperfections in the metal surfaces.

 

The process you have been shown .........

Its all about how you do your services and repairs too.

If you dont do a good job you will have ............ this eventually happen.

 

Wow ! Nice picture.

 

I think your premise is faulty- most soda bottle caps do have a flat, compressible rubber-like ring or disc inside that seats against the lip of the bottle top, acting as a barrier against liquid escaping. Unscrew the cap 1/2 turn and try holding it upside down or squeezing it again (but not while I'm nearby!).

 

Bingo. The waxy lid bottom is the sealing surface.

We can calculate pressure drop at flow rate across connected threads. Using close tolerances and many threads per inch, threads without any elastomer seals cannot do better than about 95% of a pure vacuum. Such threads and total sealing length cannot be used in a watch crown or crown tube. Therefore to seal to 1 atmosphere the watch design simply must include an o-ring of a flat gasket.

 

No matter how precise a tread is, it will never seal 100% against moisture/humidity/water. Even if you have a plastic cap on the bottle, that will not seal properly unless there is a flat rubber/styrofoam pad inside the cap that seals directly onto the lip of the bottle. And that can be compared to the caseback/push-pull crown of any water resistant watch. However, there are cases when a watch has a snap on case and push/pull crown and still has a 200m water resistance rating...but not for diving.

 

There is a gasket in the cap of a carbonated beverage bottle. If you look carefully at the underside of the cap, there's an EVA insert that seals against the top of the bottle.

It's not the threads that keep the carbon dioxide for escaping, it's the EVA seal.

Myles

 

This is the crown seal design used on the 7S26-0020 (e.g. SKX007) and 8L35-0010 (Marine Master) models. The crown screws down, and the sole seal is an o-ring on the crown "shank" that seals against the inside of the case tube.

Unless the case tube tapers, screwing the crown down has no effect on the seal. Rather, it only serves to immobilize the crown and help protect it from impacts. If the case tube does in fact taper internally, then screwing the crown down compresses the o-ring to create a tighter seal. Does anyone know if the case tube is tapered?

Best wishes,
Myles

 

I always thought the purpose of a single gasket screw down crown was to prevent the crown from opening inadvertently while under water.

 

I always understood the purpose of a screw down crown was to prevent the crown from getting ripped out by being caught on an some underwater obstacle, a reef, a rock or a wreck.

 

That is/was my understanding, a watch maker I know has repaired many Rolly Subs he was telling me that guys had set the time and forgot to screw the crown back down before going in the sea/pool.

 

That should be referring to the crown guard.

 

The screw down crown came before the crown gaurd.

 

HPN, isnt that a ring of softer grayer plastic inside the cap? it looks it from the photo... With the longitudinal grooves in the threads its even clearer that the threads alone cant maintain a pressure differential, but they do supply the compression of the seal.

 

Yes, I can see it looks that way, but it's just the way the light's
caught it in the scan. The cap is all the same colour and hardish plastic material - can't compress anything by hand at all. What looks like greyer
plastic is just a shoulder( if that's the right word), part of the same
one piece moulding and sits inside the bottle opening.