Amplitude is the name of the watch-adjusting game. We need that balance wheel to swing if we hope to adjust the watch to keep good time in several positions. As we’ve explained earlier, small poise errors cancel out at around 220 degrees of amplitude. To get that, we need a watch to have at least 270 degrees of amplitude in the dial up and dial down positions. So we’ll do whatever it takes to crank up the amplitude, short of provoking “knocking.”
If you aren’t doing so already, you should test the freedom of your mainspring barrel after cleaning the watch but before inserting the mainspring. A reasonably common problem is that the barrel arbor binds against the barrel. This is bad. Amplitude starts at the mainspring barrel, so if it isn’t spinning freely around its arbor, you’ll never get great amplitude.
To check the barrel’s freedom, simply close up the barrel, hold the arbor, and see how well the barrel spins. Bergeon makes an expensive, fancypants tool to hold the arbor, but you can just use a pin vise.
For example, here’s the assembled mainspring barrel for a Russian ZIM 2602 wristwatch. The top of the arbor is held in a balance screw holder, and I test the barrel’s freedom by seeing how easily it spins.
The barrel should spin easily when flicked or puffed with a blower. This one, though, barely moved at all. Such high friction would stunt our amplitude, and that’s bad.
Most Likely Reason: Gunk
Barrels can seize on their arbors for many reasons, but the most common one is gunky grime. Vintage watches have had old oils and greases congealing in their barrels for decades. The crust on the arbor and barrel holes isn’t always removed by a cleaning cycle.
So, it’s back to the old school methods. Pegging out the holes with a toothpick will scrape away old grease and polish the hole. The arbor’s shoulders should be scrubbed as well.
And that was all it took for this watch. When I reassembled the barrel, it spun freely and smoothly—it’s ready for the new mainspring.
Less Likely Reasons: Poor Sideshake or Endshake
In nearly all cases, cleaning the barrel and arbor does the trick. If your barrel still doesn’t spin freely around the arbor, there are some good steps to troubleshoot the problem.
First, take apart the barrel and stick the arbor ends into their respective holes. If both arbor ends stick in their barrel holes, it’s possible that the arbor (or barrel) is an incorrect replacement that got swapped in at some point. Such a problem is common for watches from back before parts were truly interchangeable.
More likely, though, the arbor sticks in only one end and not the other. If only one hole is too small—that is, the sideshake isn’t enough—you can slightly open the hole using your staking set.
Finally, I most often find that the arbor doesn’t stick in either hole yet binds when the barrel is assembled. This is a problem of endshake. The barrel is slightly distorted, causing it to pinch the arbor.
In The Watchmakers’ Staking Tool, Archie Perkins discussed a couple ways to increase the endshake (see page 39 of the book).
Most likely, the snap-on cap is cupped inward. If so, get a flat-faced hollow stump and a round-faced solid punch.
Just tap the cap to increase the endshake and reduce the inward cupping.
Alternately, you can assemble the arbor in the barrel and get a flat-faced hollow stump and flat-faced solid punch.
Just tap against the arbor in the direction that would reduce cupping.
Testing if the mainspring barrel spins freely takes just a moment and rules out one of the most vexing causes of low amplitude. If you don’t do this and get a watch running at 230 degrees dial up, you’ll run around in circles trying to troubleshoot the problem.
Good amplitude starts at the mainspring barrel, so scrub and spin that sucker.