Mainspring Sizing

[Lawren5]

I need to find a replacement mainspring for a Ball pocket watch which uses the Hamilton 992B movement. The broken mainspring measures 2.82 mm wide, 0.16 mm thick/strength and 23.5 inches long. The closest replacement I can find has all the right specifications except that it is only 20.75 inches long, nearly 3 inches shorter. Would using a short mainspring present any problems?

[JohnR725]

finding a mainspring for a 992B of the correct size shouldn't be that hard as it's relatively common. then the Hamilton material book indicates the mainsprings of 534B with the thickness of 0.155

[Lawren5]

I don't think the 534B will work because it has a width of 3.0mm whereas the broken mainspring measured 2.82 mm. The 534-718047 with a 2.80mm width is the one I've been considering and should work if the shorter length is not a problem.

[JohnR725]

what was bothering me in the back of my mind is I worked on a ball watch and there were some differences and I just can't remember what they were? then before you remove the mainspring did it seem like there was additional space in the barrel for something wider? It's unlikely that they made a custom mainspring for the watch so it would have to be one of the Hamilton mainsprings. so I'm giving you the complete Hamilton 16 size section so you can see what once was available and what is currently available hopefully.

[Lawren5]

It didn't look like there was additional room in the barrel. Also, the barrel cover is thicker in the middle than it is on the edge so it would almost certainly make contact with a 3.0 mm mainspring.

[JohnR725]

usually the thicker part of the barrel lid isn't a problem because if you put the arbor in the lid that corresponds to that diameter.

I have an image from the 1946 Hamilton material catalog. Basically were down to they used the mainspring for the Hamilton 992 not Hamilton 992B. You notice in the catalog here when it came to end style both the 534 and the 534B are for motor barrels. 

so looking at eBay searching for 992 mainspring keeps coming up with 992B mainspring but I did find a couple in 992 and at least one of them is a white spring which typically makes it newer. The older blued spring is depending upon who made them can hold up and be very nice condition or they can be set and occasionally they get rusty not always noticed by the seller.  Of course getting the white spring means that while you have a whole bunch of choices a very long time ago typically on the modern spring they've consolidated all those choices down to 1  maybe 2  if you're lucky.

 

[Lawren5]

Thanks, John. I appreciate your comments.
The thicker part of the lid extends almost to the edge so it would be a problem. I'll order the 534 because I know it will fit.