Manual mainspring in automatic watches

Update (2025-06-02): So it turns out these are called two-piece automatic mainspring and are not uncommon for early automatic watches. I invite you to read on to follow my learning journey, despite I may not have been correct the first time.

The mainspring of a manual and automatic watch work differently.

Lip of a manual watch mainspring hooks onto a notch on the barrel wall which holds it in place as user winds the watch.

As the spring is wound it presses against the notch, until the spring cannot be ound further.

Automatic watch mainspring differs by not having a lip at the end or notch on the barrel, and instead another strip of spring forks out to tension against the loop when the spring is nearly fully wound.

This way, when the mainspring is nearly fully wound, the end would push out and begin slipping along the barrel wall, to effectively unwind a bit. Otherwise, when the winding weight turns against an unyielding mainspring, gears on the automatic works would need to absorb that stress and eventually cause breakage.

Therefore it seems that a manual mainspring that stops dead when fully wound is not suitable for an automatic watch.

However in my watch repair journey I have encountered 2 examples of automatic watches that had their barrel converted to accept mainspring without the strip of additional spring like an automatic one.

Our first example is a Seiko 603 from a gold-capped 20j Seikomatic.

When removing the mainspring from the barrel, to my horror the mainspring simply flied out and 2 stips of spring emerged.

Upon closer inspection, and comparing the mainspring against an example the fettling guy has posted online, I concluded that the spring on this Seikomatic isn't a mainspring for automatic watch.

But how could that have worked?

Here the shorter stip, which appears to be of similar length as the added strip an automatic mainspring would have, has 2 notches.

Its purpose is apparent when I install that into the barrel.

With that hook at the end, it starts to look like a manual watch barrel.

So when I wind the S-shaped mainspring, which happens to look almost like a manual watch mainspring, it fits just fine.

And when I put some wind into it by turning the screw above the rachet wheel, the movement runs.

Of course, since the watch is missing the automatic spring strip at the end, when the watch is fully wound the stress caused by automatic winding weight will be exerted on the pawl lever and the transmission wheel, which happen to have very thin arms and tooth, so they will easily break down the line.

It looks like a bandaid measure to keep the watch running without the need to source a new, proper mainspring.

To that end, it has achieved its goal.

The second example came from 7625 automatic movement on a Seiko Sportmatic Calendar 820.

A 7625 is a 7622 (previously 860) with its manual winding facilities replaced with automatic winding.

The barrel looks dirty as usual. Some dried on old oil has turned dark.

When I attempt to remove the mainspring, I thought it'd come out without contest like the one I found on another 7625. However this time, it jumped out on me like the 603 one.

For the first 2 seconds I thought the violent unwinding of the spring has caused its automatic strip to break off until another thought jumps out—don't the symptoms look quite like the case of the 603?

Well, there are 2 hooks so…

Like in the case of the 603, this 7625 mainspring has been modified to form a hook and work like a manual winding mainspring.

Comparing the 7625 barrel fitted with modified mainspring (white background) and a 7625 barrel fitted with proper automatic mainspring, it's obvious that there are more gaps and the outer strip can be seen wrapping around the inner strip.

Unlike the 603 modified mainspring, this one does slip like an automatic mainspring. Maybe because I didn't apply braking grease to the 7625 barrel wall.

However, it is slipping rather prematurely causing lower amplitude and impacting isochronism of the watch.

It will not run well until a replacement mainspring or braking grease has been applied. I don't want to encounter another mainspring went flying incident though, these mainsprings never come out gracefully.

I have searched online and have yet to encounter any example of this treatment to a mainspring.

Yet I can easily piece together why the mid-century watch repairers opted to do this. We now live in an age where replacement parts can be easily found, bought and shipped whereas during the 20th century it takes a longer time to do the same, and the repairer was under time pressure to deliver the watch to its owners.

If a broken automatic mainspring is found, it would only take an afternoon to bend the short strip into shape and put back into the barrel. This improvisation gave the broken mainspring another life, and in this case, decades of lives until they reach my desk.

And since it's rather expensive and troublesome to hunt for a suitable replace, they probably will go on to drive the watches for more years to come.

Update (2025-06-02) cont'd: That's all I thought were true, until…

Apparently early automatic mainsprings are actually constructed in two pieces: a slipping bridle (the shorter one) would be inserted into the barrel first, making a hook for the manual mainspring to catch on.

When the spring force becomes too great while winding, the slipping bridle will slip along the barrel wall and in effect functioning as an automatic mainspring.

In the case of the 7625 mainspring, it indeed was properly slipping. I should have added braking grease to prevent it from doing so too prematurely though.

And in the case of the 603, I may have added too much braking grease (at all 6 spots), while the recommended amount was 5.

Below are some sources I found by searching the proper keyword "two-piece mainspring" (shouldn't have trusted AI that much):

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