Adding weight to a balance wheel

[Barnaby]

I am working on a Felsa 302 (1950s ladies movement).

I've installed a NOS balance complete, and fully serviced the movement. It runs 15-20 mins fast per day, even with the regulator at it's furthest extent. Yes it's been demagnetised.

 

Normally at this point I would either unscrew the timing balance screws and/or add timing washers. However on this balance none of the screws have slots for a screwdriver.

 

So how to add weight to the balance?

[nickelsilver]

You would use balance screw holders, which are small sliding pin vices with precise diameter openings. Grip the head and unscrew. They are honestly an essential tool for slotted screws as well; get the screw loose with a screwdriver, then unscrew with the holder, now it's held and easy to add a washer to and get it back on the rim, final tighten with screwdriver.

 

The standard set is 3 pieces, 0.30, 0.40, 0.50mm. That covers things pretty well, unless you're working on a tiny 5 ligne LeCoultre or a big pocket watch.

 

Do try to eliminate any other cause of the increased rate before going to adding washers though.

 

 

Edited June 3, 2022 by nickelsilver

[oldhippy]

Are you sure it has the correct mainspring? if so make sure everything to do with the balance is correct. Look at the hairspring to see if you can let it out. 

[Nucejoe]

How about a slo mo vid of the hairspring coil when running?  Intermitant sticking does sometime go unspotted with naked eye.

[Barnaby]

The mainpring is correct, and the hairspring isn't sticking. The balance complete is NOS and looks perfect when running- no wobbles, hairspring flat etc...

So I think the balance screw holder is the way to go.

Thanks for your help.

[JohnR725]

On 6/4/2022 at 2:01 AM, Barnaby said:

The balance complete is NOS and looks perfect when running- no wobbles, hairspring flat etc...

new old stock is always a peculiar term? Normally the factories would get the timing much much closer than what you're seeing.  it makes me wonder if someone had balance wheels with separate hairsprings and put them together because they could and that's not the way they normally would've been. Normally the hairspring is vibrated for specific balance wheel.