So here it goes, there are a number of ways to set a good gear mesh. Car and truck drivers may have heard of the "paper method," in which a piece of notebook, printer, or even cigarette paper is cut into a small strip and placed between the two gears being pushed together. With the paper strip pressed tightly between the two gears, tighten the adjusting screws down and remove the paper. The paper should be pressed into a zigzag shape from the teeth, but not be cut through. If the paper is cut through, the mesh may be too tight. This method works for many applications and has been proven as a tried and true system. But it is important to check the gears regardless of the method used. To check gear mesh, hold the smaller gear of the two and move the other gear back and forth. A small amount of movement--very small--should be present. This movement is called backlash, the movement of the teeth of one gear inside the gaps of the other. This movement should, in most cases, be very small and just perceptible; anything more will be too loose. If no movement is present, the gears are too tight. This is where "feel" comes in. With time and practice, a pilot will know what the proper amount of backlash feels like. It is very helpful to have an experienced pilot set it the first time so that you can get a "feel" for it by moving the gears. Using the paper method will get very good results until this "feel" can be obtained.
It is also important to note that since most gears are not perfectly round, the mesh needs to be checked at various points around the gears. If there is a high point in a gear where the mesh is tighter than the rest of the circumference, it may work best to set the mesh tighter than normal at that point, so that mesh will be right around the rest of the gear, the high point should wear in over a short time.
All gears should be set with a small mesh, with as little backlash as possible (but with no drag or tightness). Another way to check if things are too tight is to just spin the gears and listen; they should spin freely with little noise. If there is a grinding noise when the gears are spun, the mesh may be too tight. Use shims or spacers wherever necessary to get gears nicely meshed together.
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