Advice on choosing a new crystal

[Bill2024]

I have an old Rotary watch which is my first project. One of the many things it needs is a new crystal but I’m unsure of what to buy. I have attached a picture of what I need to replace. 

The old crystal has a lip around the bottom edge. Is this what Sternkruz calls a crystal with a step?  They also seem to sell snap-in crystals and crystals without a step. Can anyone suggest which type I might need please?

I have looked at the Sternkruz catalogue pdf from the cousinsuk website but the images are not really clear enough to make out which type I need. 

It also looks like the crystal is a high dome one but can anyone confirm from the pictures?

Also I measured the watch case where the crystal fits and it measures 30.4  Is that the size I should order, and also maybe one size bigger?

EDIT: I just looked at the crystal again and actually it seems to have sloped sides rather than an actual lip along the bottom edge. New image attached .

thanks, Bill

Edited February 11 by Bill2024
Added new image of crystal side profile

[AndyGSi]

I thought what you were describing at first is an underlayed crystal but these are designed to have a bezel over them.

Looking at your last photo it looks like a high dome with step but you may have a problem getting one with such a high dome.

What are the dimensions?

[Bill2024]

1 hour ago, AndyGSi said:

What are the dimensions?

It’s 30.4 by approximately 4.8. 

It’s a bit tricky to measure accurately as it is cracked. Also I can’t be sure it is the original crystal. 

Edited February 11 by Bill2024

[AndyGSi]

HWS 30.4 has an internal height of 3mm and an overall of 4.4mm

HW 30.4 has an internal height of 2.8mm and an overall of 4.3mm

There's also the HH which has a similar profile to the HW, I've not got one but the catalogue shows an internal height of 3.7mm

Edited February 11 by AndyGSi

[fixermole]

Bill,

As a general rule of thumb for acrylic crystals, you order one that is 0.2 mm larger than the case opening where it fits. Use a Bergeon 4266 crystal lift, or equivalent, to install. So, if the opening is 30.4 mm in diameter, you order a 30.6 mm crystal. I can't comment on the height, but for most vintage watches I default to the high dome step ones (Sternkreuz HWS).

[AndyGSi]

If the existing crystal is 30.4 then the allowance mentioned by
@fixermole should already have been taken into consideration.

[Kalanag]

But don‘t touch the dial with your caliper, as shown in the video (1:51)!

Edited February 12 by Kalanag

[Bill2024]

6 hours ago, Kalanag said:

But don‘t touch the dial with your caliper, as shown in the video (1:51)!

Thanks that’s useful. I noticed they say for tension ring crystals to go up by 0.1mm for the crystal size. But for non-armoured crystals it seems that a crystal 0.2mm bigger than the measurement should be used. 

I can’t use their method for measuring the height as I have removed and dissembled the movement. It does seem risky to put your metal callipers on the dial though!

[AndyGSi]

3 minutes ago, Bill2024 said:

Thanks that’s useful. I noticed they say for tension ring crystals to go up by 0.1mm for the crystal size. But for non-armoured crystals it seems that a crystal 0.2mm bigger than the measurement should be used. 

I can’t use their method for measuring the height as I have removed and dissembled the movement. It does seem risky to put your metal callipers on the dial though!

Always best to work on the case without the movement.

I don't see why the 30.4mm you've measured from the original won't work as a replacement.

Can you measure the internal of the existing.

[Bill2024]

7 hours ago, AndyGSi said:

If the existing crystal is 30.4 then the allowance mentioned by
@fixermole should already have been taken into consideration.

Yes I think you’re right. I suspect I have been measuring the existing crystal a little narrower than it was because it is so cracked.

edit: the case is definitely coming in at 30.43. So I think I’ll try a 30.6 crystal in the various styles you showed pictures of in a previous post. As well as understanding a bit more about how the crystals fit I’d like to what the visual style of the different types look like. 

Edited February 12 by Bill2024

[AndyGSi]

27 minutes ago, Bill2024 said:

As well as understanding a bit more about how the crystals fit I’d like to what the visual style of the different types look like. 

Not that easy to photo as you can see above and always good to have a reference to work to for future projects.