Mainspring for Tissot cal 27

[rogart63]

Looking for a mainspring for an old Tissot cal 27 movement. Ranfft says 1.30 x 10.5 x 0.12 x 320mm , But the closest i could find on cousinsuk was 340 mm . But when measuring the old it's 0,13 and Jules borel says  1.30x 11x 0.13x 340. Which one would be best? 

As the mainspring is of old standard i think 0,12 will do? But what do you think? 

[clockboy]

Using my "General Resorts"  ref: book spring is GR3515

Cal. 27 1.35 x 10.5 x 0.12 x 340mm  ⦱10

 

HOWEVER there are many cal 27,s listed but followed by a letter so be absolutely sure it is just a cal. 27

 

IE Cal 27 M spring is GR3338

Cal. 27 M 1.35 x 10.5 x 0.13 x 320mm  ⦱10.5

 

[praezis]

Length 320 or 340 does not matter at all.

0.12 or 0.13 does. With 0.12 you may have a slightly lower amplitude but will avoid knocking (esp. after a good cleaning).

Frank

[rogart63]

5 hours ago, clockboy said:

Using my "General Resorts"  ref: book spring is GR3515

Cal. 27 1.35 x 10.5 x 0.12 x 340mm  ⦱10

 

HOWEVER there are many cal 27,s listed but followed by a letter so be absolutely sure it is just a cal. 27

 

IE Cal 27 M spring is GR3338

Cal. 27 M 1.35 x 10.5 x 0.13 x 320mm  ⦱10.5

 

I am absolutely sure it's a cal 27. Only says 27 on the mainplate. But little strange looking at the size for that GR3338 on cousinsuk. It's says 1,30 instead of 1,35 . 

But the last  will be perfect. As i think that is the closest to the original size i could find. And hope it isn't to strong? As the watch is from around the 1930-1940. Think we dated it to 1939 with the serialnumber.  

 

Edited April 17, 2017 by rogart63

[rogart63]

5 hours ago, praezis said:

Length 320 or 340 does not matter at all.

0.12 or 0.13 does. With 0.12 you may have a slightly lower amplitude but will avoid knocking (esp. after a good cleaning).

Frank

Maybe i should go for the 0,12 as the new mainsprings are much better then the old ones where? At least as the watch is from 1939. 

[clockboy]

10 hours ago, rogart63 said:

I am absolutely sure it's a cal 27. Only says 27 on the mainplate. But little strange looking at the size for that GR3338 on cousinsuk. It's says 1,30 instead of 1,35 . 

But the last  will be perfect. As i think that is the closest to the original size i could find. And hope it isn't to strong? As the watch is from around the 1930-1940. Think we dated it to 1939 with the serialnumber.  

 

When I searched Cousins for a Cal 27 mainspring it gave GR 3515 which is the same as the GR book suggests. If it was me thats what I would go with because it is longer so you will get a better standby time.

[StuartBaker104]

There is more here than I will ever need to know about mainsprings...

http://www.vintagewatchstraps.com/blogmainsprings.php, but I also read somewhere once that the height of the spring should be 0.2mm less than the height of the space in the barrel, although this is not easy to measure in practice.

The GR catalogue lists 2 different sizes for a cal 27; on the last page of this link https://www.cousinsuk.com/PDF/categories/7813_GR Pages 171 - 180.pdf and the first page of this one https://www.cousinsuk.com/PDF/categories/7814_GR Pages 181 - 190.pdf. Cousins parts finder for the 27 recommends the GR3515

In practice I would follow the advice which Frank gives above.  Modern alloy springs tend to give slightly more power than their historical steel counterparts. So where you have a choice, go for thinner raher than thicker.  The possible length is then an outcome of the available space per the first link above, and a longer spring will give more power reserve.