Advice on choosing a mainspring

[MikeAdvicePI]

I'd like to ask for some advice about choosing a replacement mainspring. I'm servicing a Union SA pocket watch and I want to replace the mainspring.  I think the movement is a Unitas 361 movement. The layout and dimensions agree with Dr Ranfft's description of a 361.

 The mainspring I've removed from the barrel measures 1.8, 0.16 and 480 long. The internal diameter of the barrel is 16mm. The mainspring has a hole at the barrel end and the barrel has a hook.

Ranfft's description of the movement says the mainspring dimensions are 1.9, 0.195 and 500 long for a 16mm barrel.. The Generale Ressorts catalogue lists the spring for a Unitas 361 as No 5650 and its dimensions are the same as the spring Ranfft lists. Cousins lists a 5650 with those dimensions.

However, that 5650 spring has a conventional non-automatic barrel end rather than the end with the hole. Hmmm. There doesn't seem to be a similar sized spring designed for the barrel with the hook. Of course, the barrel may not be the original one.

If the new 5650 spring is likely to hold against the hook in the old barrel, I would go ahead and try it. That's what I'm tempted to do, unless someone knows it would be a daft idea. What do you think I should do next? Try the 5650 spring? Try the closest sized spring with a hole? Put the old one back in and move on? All advice welcome.

[oldhippy]

Is there something wrong with the mainspring? If not use it. That hook in the barrel doesn't look right its to long and I would say any spring fitted in that barrel will not sit proper and the coil that rests next to it could be rubbing against it causing wear on the spring. As it is a conventional non-automatic barrel end it won't be any good as it won't fit snug to the barrel, if you shortened it you can use one.  

[MikeAdvicePI]

 

Thanks for that, oldhippy. Just to be clear, you mean that if I reduce the amount the hook sticks out into the barrel then the modern 5650 spring could work. If that's the case I'll go for it. I'll try to leave it so that the old spring could still be refitted. The reason for wanting to replace it, apart from the experience, is that the amplitude of the balance was very low and I was hoping to improve it.

You can see from this photo how much the old spring was displaced from the barrel wall.

I'll report back on how it turned out.

[oldhippy]

 

That's what I meant, you can see how it would rub and it doesn't sit right in the barrel, you might be able to bend it away but be careful not to break it off, I myself would file it down, remember to remove rough edges. Just imagine the angel the conventional non-automatic barrel end would have, it would probably rip the end off. 

[MikeAdvicePI]

I was thinking that filing it was the safer option. The hook's been formed by punching through the barrel wall so breaking seems very possible. I don't want to risk deforming the barrel either.

I've put the order for the GR5650 spring in with Cousins (actually I've ordered two springs by accidentally putting it in my basket twice) and I'll try to remember to take pictures of the process.

[MikeAdvicePI]

I pushed the hook back close to flush with the barrel wall and filed the end to create a stop for the tail of the mainspring.

 

When I tried to insert the mainspring it caught on the top of the protruding hook - that is it wouldn't sit down inside the barrel at that point. I had to unwind it by hand and replace it in the barrel by hand with the tail of the spring against my newly filed stop. As that was a two hand job I didn't take any photos. However, the fitting was successful and the mainspring winds properly and holds in the barrel. Unfortunately, it didn't make a great difference to the amplitude. I suspect the problem lies with the cleaning of the train, especially the escape wheel, which appears to stop very abruptly some of the time, while the other wheels spin freely and wind slightly back before they stop.

I'll come back to this watch another time when I run out of things to do.

[praezis]

o.16 (old) to 0.195 (new) should make a big difference!

Frank