Yankee Screwdriver Hex Bit Adapter
Bits for Yankee screwdrivers can still be bought but at a price. There are also adapters on the market allowing the use of modern 1/4” hex bits but they are also expensive.
The bits were made with shank diameters of 7/32”, 9/32” or 5/16”. Now 1/4” hex bits are as near-as-damn-it 9/32” across the corners — 1/4” refers to the size across the flats. Therefore if you have a Yankee that takes 9/32” bits, such as a No 130, it’s easy to make an adapter by getting a cheap magnetic bit holder and setting about it with a file.
I’m certainly not the first person to do it but I wasn’t able to find information on the size and location of the necessary cuts and didn’t have a bit to copy. Below is what I came up with, the red areas being those to remove.
Of course it would be possible to modify long hex bits in the same way rather than use a magnetic holder. The only snag is that these are likely to be too hard to file and would have to be ground.