The outer diameter of this replacement cannon pinion, which took a while to find, has to be reduced a bit. Here it is friction fit on a piece of scrap. Frequent trial fit to the hour wheel is required. If you take off too much, you can't put it back... After the fit is nearly right, the surface is polished to a nice shine.
It is often the case with replacement parts that they aren't exactly right, particularly the older ones, even when they are supposed to be "the right part". Even during the original assembly at the factory, parts were hand-fit. And it's one the rules of this sort of work that we never modify other parts to find the replacement, always alter the replacement.
When you open up an old watch, you'll find 100 years worth of repairs of all sorts and all sorts of parts, often hand made from scratch. I am sometimes asked by potential customers if I use only factory original parts. It's hard to explain the many reasons that this question doesn't even make sense, not the least of which is that the factories that made these parts have not existed for decades or longer.
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