About 2.75" center to center
The knurled nut is 3/8-40 and has a set screw that locks it in position. This lets you set the axial play to about nothing.
The rollers are standard shouldered drill bushings that you can get right out of the MSC catalog. They are about 5/8" ID x 1" OD with a 1.25 dia flange. They are dead hard.
the shafts are turned wtih a threaded spigot that bolts to the block. I lathe cut the male threads. It's funny now to see the spiral pattern on the shaft after some use...I didn't see that after it was made. Make the fit here nice and close. Additionally, you don't need or want much in the way of lubrication. A shot or Starrett M1 lubricant once a year is all it needs or the drag from the oil film will make the file skip.
You need a safe edge file to use this ( no teeth on the edge)
Keep filing until both rolls are turning and the file is not cutting...and your there.
The dovetail was duplicated from one of my holders
The micrometer and bracket were from some yard sale...came just as it is....drilled and tapped two 8-32 holes and it was on and fit perfect.
Knock the corners off so you don't run your hand into sharp edges while your filing.
The body could be made in one piece, but I had the two blocks in the lucky box.
Dave