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Lead screw pitch

I believe that the Jennings bits have two different threads available on the lead screw.  A fine thread for hardwoods and a coarse thread for softwoods.  Most sellers on ebay don’t seem to know the difference or simply don’t have anything to compare with to determine whether it is fine or coarse.  The Irwin bits have a thread that is somewhere in between the two, more of a general purpose one size fits all arrangement.

The other thing I can tell you about Jennings augers is that they don’t clear chips as well as the Irwin augers in deep holes.  The Jennings bits have two flutes that extend the entire length.  I believe they are made from flat bar that is twisted into a spiral.  Irwins have two flutes for the first revolution or so then one of them just stops and a single flute extends the rest of the way up.  This gives much more space for shavings to escape.

If I had to pick one or the other, I would probably lean toward the Irwins based on price and availability.  Plus, they’re pretty good tools.  Especially since you have a partial set already and just need to fill in a couple of missing sizes.  It would be cool to keep using your Grandpa’s tools.