Expert: Martin Meyer - 8/19/2007

Question
I have a 340-020 clock that erratically stops. Upon my inspection I see the first gear shaft after the mainspring is very sloppy.

Is there a place where I can procure the correct bushings and drill that will permit me to repair this.

Thanks in advance:
Dale in Michigan

Answer
Hi Dale,
    This is a much more complicated matter than it appears.
    To answer your specific question, clock materials houses such as Timesavers and Merritts Antiques stock a full assortment of rebushing equipment and bushings.
    Now, to try to address some of the details of rebushing bearings. First, when one bearing is badly worn, it is almost a certainty that numerous other bearings are also worn. This is sometimes not visible from the outside of the movement. If you see black accumulations of "gunk" at other bearings, this indicates wear in these bearings. This material is actually a mixture of dried oil and worn metal from the bearing hole and pivot.
    I'm not sure if you are looking for a rebushing tool that will work from the outside of the movement plate, thus avoiding disassembling the entire movement. There is such a tool, I have one, and never use it. It does a very inferior job (in my opinion).
    If you are planning on disassembling the entire movement, you will have to clean all parts, either by machine or by hand, with appropriate cleaning solutions. Be warned that reassembling is a job that requires considerable experience, and some of the gears must be indexed relative to each other, thus usually requiring repeated partial disassemblies to line up the gears with each other. The reassembly of the parts on the front movement plate also requires very precise alignment and adjustment.
    One more essential task is to polish the pivots of all gears. The pivots of the gears become scored by rotating in worn bearing holes. They must be restored to smoothness and "mirror bright" finish. This is done on a watchmakers lathe with the use of several grades of very fine emery paper available from clock materials houses.
    In respect to rebushing equipment: There are very expensive rebushing tools available, but I do not consider them necessary and do not use them myself. The optimum way to do it is with a drill press and a set of bushing reamers. There is a part available which will hold the reamers, which fits in the drill press chuck. There is also a reamer holding handle, which will allow you to do it by hand, but not with much accuracy. The bushings themselves come both in assortments and packages of individual sizes. For probably (just guessing) under $100 you can purchase an acceptable set of tools and bushings (not counting the drill press and watchmakers lathe, of course). Drills are useless for rebushing, they do not provide anywhere near the precision required.
    Please understand that I am not trying to make this all sound overly difficult or to discourage you from doing it. It's just that it is a somewhat difficult operation. The bushings and reamers are designed so that the O.D. of the bushing is about .001" larger than the precision hole that the reamer makes. So the bushings are a press fit into a hole slightly smaller than their O.D. The bushing I.D.'s vary by .1mm (.0039")(smaller bushings) to .2mm (larger bushings. The ideal tolerance for the pivot in the bushing hole is .002", although you can go a little larger and get away with it. Therefore, unless you just get lucky occasionally, it is essential that the bushing hole be reamed to the correct tolerance by two sets of broaches, first a cutting broach, then a polishing broach. These sets are also available from clock materials houses.
    As you can see from these tolerances, it is also critical that the holes for the bushings, and the holes in the bushings, be perfectly perpendicular to the movement plate. Any slant in the hole will more than cancel out the tolerance required. Finally there must be a correct "end shake" in the arbor (shaft). This is a tolerance from end-to-end between the arbor and the front and back movement plates. It is not so critical as the pivot tolerance (.004" is probably very good), but it must be there, or the arbor will jam between the plates.
    I realize that I could have just recommended the basic tool that cuts the bushing hole from the outside of the plate. But I cannot bring myself to recommend a procedure that I think is totally inadequate and will not give you a successful result. I apologize if my answer is overly technical, but I must give you an answer which is professional and will really work.
    Thanks for your question.   Martin