You mean this one.....
Pinion gear mock-up:

Remove the bearing from the old pinion using a press and bearing separator, as illustrated above. Remove the shim from the pinion; measure and record the thickness. The nominal shim thickness used on an FMS gearset is .030", but can vary +/- .002". If you are installing FMS gears, look closely at the new pinion; it may be marked with a +2, +1, 0, -1, -2. If so, that "number" is added to .030" (e.g +2 would indicate the use of a .032" thick shim; likewise, the -2 would require .028") to obtain the proper shim thickness for that gearset. Regardless of the marking on pinion (which is an expeditious method of getting close to correct pinion depth), the final result should always be checked with proper tooling and marking compound.


http://www.corral.net/tech/drivetrain/gears.html