1930s Tissot Tank ID Help: CK 730, Cal. 20

ruibelfort · Apr 25, 2026

Hello everyone,

This watch recently came into my possession, and I would like to restore it as close as possible to its original condition. It is currently non-running and missing several components, including the crystal, hands, and buckle.

I have already sourced a donor movement (Cal. 20) with a very close serial number. However, I am having difficulty identifying the exact model to correctly source the remaining parts - especially the hands.

Could anyone help confirm whether the watch is original, assist with model identification, or share reference images/catalog entries of similar pieces?

Thank you very much in advance.

ruibelfort · Apr 26, 2026

Some additional Info.

Movement:
CAL.20

Serial:
778.428

Box engraving (internal):
METAL
PLAQUÉ D'OR LAMINÉ
GARANTI 10 MICRONS

TISSOT
FAB SUISSE
SWISS MADE

RC
672314

Box engraving (external):
ANTIMAGNETIQUE

Vitezi · Apr 26, 2026

Hello @ruibelfort and welcome to the forum! Your watch looks entirely original to me, including the KB case and dial. It would originally have had gilt leaf hands (gilt hands to match the gilt numerals) like this jumbo version of your tank:

ruibelfort · Apr 26, 2026

Thank you, @Vitezi !

Would you have any guidance on correct hand sizing for this model?

The visible dial area is 30 mm x 20 mm. Based on my research, I arrived at the following estimates:

Lengths (center to tip):
- Minute: ~10.0 mm
- Hour: ~7.4 mm
- Small seconds: ~3.3 mm

Hole sizes:
- Hour: ~1.20 mm
- Minute: ~0.70 mm
- Seconds: ~0.25 mm

Do these proportions sound correct to you for this reference?

ruibelfort · Apr 26, 2026

Looking through other discussions here I found this references and it seems the model is a CK 730 20 equivalent (thank you @Tire-comedon for sharing this material).

DON · Apr 27, 2026

Find a local watchmaker if you can. Leaf hands are a dime a dozen and any long time watch maker should have in stock.

Crystal. Might have to order that. Would need the length x width. Guessing glued in. I rarely dealt with rectangular models

If you post where you are. Maybe someone can help

ruibelfort · Apr 27, 2026

Thank you @DON

I’m based in Recife, Brazil. I work with an excellent local watchmaker, but sourcing period-correct parts is still a challenge.

Most components come from eBay, which I source myself to ensure consistency with the original piece.

Thanks to the guidance here, I’ve already secured the hour and minute hands. I’m now focused on finding the correct sub-seconds.

For the crystal, I found suitable replacements on AliExpress (30 × 20), but I’m still looking for a proper NOS option.

I really appreciate the support; it’s been instrumental.

DON · Apr 27, 2026

ruibelfort
Thank you @DON

I’m based in Recife, Brazil. I work with an excellent local watchmaker, but sourcing period-correct parts is still a challenge.

Most components come from eBay, which I source myself to ensure consistency with the original piece.

Thanks to the guidance here, I’ve already secured the hour and minute hands. I’m now focused on finding the correct sub-seconds.

For the crystal, I found suitable replacements on AliExpress (30 × 20), but I’m still looking for a proper NOS option.

I really appreciate the support; it’s been instrumental.

Finding an original crystal will be difficult and will add no value to the watch. If the case is flat where the crystal sits. Will be much easier to buy a generic or Google search on crystal dimensions and see what turns up.

For the small second. Blued steel or gold plated. Find something close. Tail or no tail. Problem is tube length. Too long. Needs to be filed down and cleaned up. Too short. Buy another. Watch maker should have something. Old school guys always have them.

skand · Apr 27, 2026

I have a Tissot from the same period with the same movement, but it features Breguet numerals and these hands.

ruibelfort · Apr 27, 2026

Beautiful, @skand !

Any chance you have the serial number? I’d like to check the production year.

skand · Apr 27, 2026

ruibelfort
Beautiful, @skand !

Any chance you have the serial number? I’d like to check the production year.

I have the service card at hand and can provide the numbers: case number 848650, movement number 744714, and calibre 20.

ruibelfort · Apr 27, 2026

skand
744714

Thank you. Amazing, really near the period of manufacture.

myouss · Apr 30, 2026

Vitezi
Hello @ruibelfort and welcome to the forum! Your watch looks entirely original to me, including the KB case and dial. It would originally have had gilt leaf hands (gilt hands to match the gilt numerals) like this jumbo version of your tank:

I had no idea these were a thing. Pretty cool.

ruibelfort · Jun 13, 2026

Just a quick update on this restoration: I was able to find all the pieces I needed to restore this watch as close as possible to its original state.

I found a donor movement at a reasonable price on a marketplace here in my country. It came with two of the missing hands, and the serial number is very close to my watch's. I was also able to find a NOS seconds hand for this exact model on eBay.

Last week, I left the watch with my trusted watchmaker, and in a month or two it should be ready. I'll come back here with the final result as soon as I have it in hand.

Thank you again for all the help!

TIMElyBehaviour · Jun 13, 2026

Absolutely beautiful watches shown in here.

I hope you find the parts you need soon so you can enjoy it.