If you're dealing with used gears, using the factory specs for setting up new gears is a crapshoot. You're likely to get a gear whine unless you happen to hit the exact backlash the gears have been running previously. They establish a wear pattern over decades of use, and trying to use the original specs seldom, if ever gets them back into the previous alignment they're used to running. I always try to do a tooth contact pattern with white lead or Prussian Blue before disassembling a used gearset, photograph the pattern, and try to duplicate it on reassembly. Original specs will get you into the ballpark on used gears, but they're a long way from hitting a home run at times. A couple of thousandths of backlash difference on new gears probably won't cause any problems.
Jerry
