Issue with Seiko 4006 Barrel Arbor and Mainspring Fitment

[TimepieceTinkerer]

Hello everyone,

I recently encountered an issue while working on my Seiko 4006 movement and could really use some advice from those with experience in watchmaking.

I purchased a new mainspring from Cousins GR2378X and attempted to install it straight into the barrel without success. Subsequently, I used my Bergeon NO7 Left hand winder and managed to successfully place the mainspring into the barrel.

As the original barrel arbor was missing, I had already ordered a replacement from Cousins (part number 213805). However, I’m now facing a problem: when I insert the new arbor into the mainspring, I can’t get the barrel lid to close—it simply won’t fit.

I've taken several photos to illustrate the issue, which I will share for better clarity.

I know I must be doing something wrong, but I’m not sure where. I’d really appreciate it if someone with experience could shed some light on what might be going wrong.

Thanks in advance for your help!

Best regards,

TT

[AndyGSi]

In what way won't it fit?

Is it the outside or the arbor hole?

Spring looks raised between 9 & 12?

[TimepieceTinkerer]

Thanks for the reply!

The problem is that the barrel arbor doesn’t stay centered inside the mainspring. When I insert it, it feels like it doesn’t catch properly, and as a result, it sits off-centre. Also, on the other side of the barrel, the arbor’s pivot doesn't protrude far enough—it should stick out long enough to engage and keep the full barrel assembly stable, but it doesn’t.

You can see this clearly in the photo.
Thanks for pointing out the raised section of the mainspring between 9 and 12 o’clock—I hadn’t noticed that earlier. I might be able to sort that out, but I’m not sure it’s the cause of the problem I’m facing with the arbor fitment.

Appreciate any further suggestions!

Thanks again,
TT

[AndyGSi]

Having another look and the arbor doesn't look right.

Are you are you ordered the 213.805?

[TimepieceTinkerer]

I ordered the Seiko 213.805 barrel arbor from Cousins. They don’t have a photo on their website, but the part number is SEI213805 if you’d like to check it. It’s described as “Seiko 213.805 Barrel Arbor.” However, if this is supposed to be the correct arbor for the 4006, the one I received is completely different.

[AndyGSi]

Looks like Cousins have got it wrong with their labelling/stocking

[TimepieceTinkerer]

Apparently this the case. 

Thanks 

Update on Seiko 4006 Barrel Arbor Issue

Just a quick update for those who kindly offered their help—thank you again!

I’ve now confirmed that the issue was due to an incorrect barrel arbor. I ordered the Seiko 213.805 barrel arbor from Cousins (part number SEI213805), which was listed as compatible, but after comparing it closely and contacting Cousins, they confirmed that the part was not correct for the Seiko 4006. It turns out the wrong item had been listed or picked.

That explains why the arbor didn’t seat properly or extend far enough to hold the barrel assembly securely.

I appreciate everyone’s input—it really helped me think through the problem more clearly.

Thanks again for your support!
TT