Alternative Mainspring Options for Seiko 4006

[TimepieceTinkerer]

Hello everyone,

I’m currently working on a Seiko 4006 movement and unfortunately damaged the mainspring while attempting to wind it. I've been searching for a replacement but haven’t had any luck so far.

Would anyone happen to know if there are compatible alternatives or aftermarket options for the 4006 mainspring?

Any guidance would be greatly appreciated. Thank you in advance for your help!

[AndyGSi]

Edit

There are also original ones if you do a google search and depending where in the world you are.

Edited April 12 by AndyGSi

[TimepieceTinkerer]

Thank you very much for your advice — I really appreciate it.

I'm based in the UK , and I actually purchased the one you suggested from Cousins. However, I noticed that the shape of the new mainspring is slightly different from the original one. Do you think that would be an issue, or is it generally acceptable as long as the dimensions match?

Thanks again for your help!

[AndyGSi]

Not sure what you mean about the shape being different, do you mean the new one is an S with integrated bridle, can you post photos.

[Curare]

I restored a 4006 a couple of years ago, I broke the original mainspring trying to fit it on a mainspring winder arbour that was too small (was the largest one I had) I then purchased the GR mainspring that Andy has posted, but found the arbour hole in the GR2378X is too large for the 4006 arbour, I tried to make the spring arbour hole smaller but broke it too, that was before I learned on this site and others how to close it up properly without breaking it. The only problem then is you still have to find an appropriate winder to suit. In desperation I used a mainspring from a 7019 movement that I had plenty of with only minor dimensional differences but the arbour hole is correct, worked okay with decent amplitude and is still functioning to this day.

[Nucejoe]

Have you checked with Julesborel ?  lists barrel & barrel arbour & mainsprings. 

 

 

[TimepieceTinkerer]

On 4/12/2025 at 9:05 PM, AndyGSi said:

Not sure what you mean about the shape being different, do you mean the new one is an S with integrated bridle, can you post photos.

Thanks for the reply!

The original Seiko 4006 mainspring is a T-end (or tongue-end) type, which locks directly into the arbor. However, the one I bought from Cousins — the GR2378X — was a slipping bridle type.

While I was carefully pushing in the arbor, the stud on the GR2378X suddenly popped out. I then tried to wind it back into the barrel using my mainspring winder, but unfortunately, neither the No. 6 nor No. 7 Bergeon winders were the right fit for the job. The attempt failed, and I ended up breaking the new mainspring.

I did not take any picture of the broken mainsprings.

Let me know your thoughts — especially if you’ve worked with this GR2378X type before or have any advice on a better-fitting winder size or the correct replacement part for the 4006.

18 hours ago, Nucejoe said:

Have you checked with Julesborel ?  lists barrel & barrel arbour & mainsprings. 

 

 

Thanks for your suggestion! However, shipping from outside the UK tends to cost a lot, so I’m trying to stick with local sources to keep things within budget.

20 hours ago, Curare said:

I restored a 4006 a couple of years ago, I broke the original mainspring trying to fit it on a mainspring winder arbour that was too small (was the largest one I had) I then purchased the GR mainspring that Andy has posted, but found the arbour hole in the GR2378X is too large for the 4006 arbour, I tried to make the spring arbour hole smaller but broke it too, that was before I learned on this site and others how to close it up properly without breaking it. The only problem then is you still have to find an appropriate winder to suit. In desperation I used a mainspring from a 7019 movement that I had plenty of with only minor dimensional differences but the arbour hole is correct, worked okay with decent amplitude and is still functioning to this day.

Thanks for sharing your experience — that’s really helpful! I guess the 7019 mainspring is the same size as the one in the 7009 and 7S26, and if that’s the case, I actually have plenty of 7009 mainsprings on hand, so I might give one a try and see how it performs.

[tomh207]

Are you sure you are not confusing springs? From caliber corner and the tech sheets they have there the 4006 is an automatic, which would be the sliding bridle mainspring. It also has an alarm function which is hand wound with the crown which would much more likely be a t-end for its mainspring.

the seiko springs are usually left hand winder, do you have the correct winder? There is a lot of conversation in various places about how awkward Seiko mainsprings can be sometimes having to mix and match arbors in the winders to get a usable configuration.

 

Tom

[TimepieceTinkerer]

You're absolutely right about the alarm mainspring — that one is definitely a T-end, hand-wound type. But the mainspring for the main movement was exactly as I described earlier.

Also, I don’t have any left-hand winders, so I’ve been trying to manage with right-hand ones. As you said, Seiko mainsprings can be quite awkward — especially for someone like me, an amateur — and mixing and matching arbors to get a workable setup has been a bit of a struggle.

[AndyGSi]

42 minutes ago, TimepieceTinkerer said:

The original Seiko 4006 mainspring is a T-end (or tongue-end) type, which locks directly into the arbor.

The T End of a spring is what locks into the barrel, nothing to do with the arbor.

44 minutes ago, TimepieceTinkerer said:

While I was carefully pushing in the arbor, the stud on the GR2378X suddenly popped out.

What stud?

I don't understand about the T End on an Automatic movement and suspect someone has been here before you.

I also don't understand why you can't just insert the spring from the ring and have to use a winder.

[JohnR725]

45 minutes ago, TimepieceTinkerer said:

T-end (or tongue-end)

You want to be careful T end is not the same as tongue end. I'm attaching an image as you can see quite a few springs have protruding parts that qualify as a T but a Seiko does not an automatic watch definitely does not

[Nucejoe]

53 minutes ago, TimepieceTinkerer said:

Thanks for your suggestion! However, shipping from outside the UK tends to cost a lot.

OMG, Donald Trump tariffs?      lol

 

 

Edited Sunday at 09:47 PM by Nucejoe

[AndyGSi]

I'd overlooked your mention of tongue end until highlighted by @JohnR725

The original would have come with a separate bridle which I presume you've removed before inserting the GR spring.

Still don't understand your reference to the arbor in relation to the end termination?

[TimepieceTinkerer]

This is the one I bought from Cousins, and I couldn’t manage to fit it in. If you compare it with the original 4006 mainspring, you’ll see exactly what I mean. 

18 minutes ago, JohnR725 said:

You want to be careful T end is not the same as tongue end. I'm attaching an image as you can see quite a few springs have protruding parts that qualify as a T but a Seiko does not an automatic watch definitely does not

Thanks, this really helps a lot.

[JohnR725]

21 minutes ago, TimepieceTinkerer said:

This is the one I bought from Cousins, and I couldn’t manage to fit it in. If you compare it with the original 4006 mainspring, you’ll see exactly what I mean. 

I think we need a clarification here. The alarm watch has two separate mainsprings.

the picture you show it appears to be a standard automatic mainspring end which should be identical to whatever Seiko has on the end of their spring for running the watch. Because the watches an automatic it does require an automatic mainspring so that the end can slip.

Then there's a separate mainspring for the alarm and it's been such an incredible quantity of time since I've ever service to one and I have zero idea what that would look like. It should probably have a tongue end as a guess because it doesn't need to slip

[TimepieceTinkerer]

Referring to the picture that JohnR725 kindly shared here — the one that originally came out of the 4006 is shown in Picture No. 1, and the one I purchased is shown in Picture No. 2, which I’m sharing below.

[AndyGSi]

There doesn't appear to be the photo of the original spring.

This should have a tongue end as shown by @JohnR725 and I would have expected a separate bridle.

 

[Curare]

Hope this helps:

Pictures of three springs from my 4006 service/resto in 2023, both mainsprings have broken arbour eyelets ##

As I said ended up using a 7 series mainspring which works okay:

4006 mainspring dimensions: GR2378X   .95 high 1.2 thick 400 long

7 Series dimensions:                 GR2377X  .95 high 1.1 thick  400 long

Photo added here:

Edited Monday at 02:36 AM by Curare
typo

[TimepieceTinkerer]

Thanks for your clear advice — I’m going to try it. The details and photos you shared are really helpful. Much appreciated!