Sometimes well meaning clock repair folks attempt to repair their own SWCC movements, and the results can be difficulty and damage. To avoid issues, here are some observations on rebushing SWCC plates, where misalignment can lead to major trouble.
It is disappointing to see movements defaced with huge reamers which leave ugly and deep burrs and large holes. And for all of that, the net result is a bushing that is unstable for a variety of reasons. Sometimes folks don’t bother returning the worn hole to center by filing. Because of the size of the hole left by the Bergeon bushing, correction of this problem is troublesome, even for a skilled person. NEVER rely on a reamer to return a hole to center. Bushing tools are simply not rigid enough to do a good job of that.
We’d prefer to see folks use KWM and arrive at a fitting hole by filing back to center and finishing off with a light broaching to size. Then of course trim the bushings to proper length on a lathe prior to insertion into the plates. It turns out that the KWM size #88 (its organic ID is .066”) is a great fit in many cases for SWCC (except for the verge bridges). That said, excessive pivot wear might require a smaller hole size, but the same height and similar diameter (or smaller diameter) should be used.
We fab our own bushings from .125” C360 brass rod; start by turning down the OD of the .125” brass rod to .098” OD (or as appropriate based on the wear) then center drill to the desired diameter after refinishing the pivot. For holes with minor wear, we sometimes start with Bergeon #B40 and ream open the hole (its OD is .098” and organic ID is .033”). This gives us a hole size custom to the particular pivot condition at hand, and a great-fitting sleeve bushing. Of course we rivet the bushing into place then finish using the methods described in our recent NAWCC articles.
It is super important to re-center the hole by filing back to center, especially on the winding ratchet wheel and the escape wheel. Not doing so will result in misalignment at the verge (one pallet face could be out of alignment with the wheel and the other in alignment), problems at the ratchet arm, or serious depthing issues between the involute drive pinion on the winding ratchet wheel and the mainspring barrel wheel.