The forward and reverse shims have the purpose of establishing backlash. The drive shaft shims establish the proper height of the pinion gear with relation to the forward and reverse gears, so that full tooth contact is exactly set. They call this relationship between the pinion and driven gears “conical angle.” Why, I don’t know. When you check backlash (AFTER you know the conical angle is right) you should pull UP on the drive shaft to simulate where the gears will be running under load, as they try to force themselves apart. If possible, secure the gear case in a fixture of some sort, upside down, so gravity holds the driveshaft in the right place. I have found that some old Mercurys have the bearing press-fit on the drive shaft, while some bearings float sort of free on it. Whether they float or are press-fit, what is important is that the drive shaft bearing is tight against the shoulder of the drive shaft, when you’re checking backlash. Also, for us people in the field, that .003 to .005 IS kind of a guesstimate. They told us that, at Merc school in 1975. At the time, there were no tools available to the average dealership to actually measure it. You can use Plasti-gauge but it is a very cumbersome, PITA process.
Long live American manufacturing!
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