I'd never noticed this but there's an error in the scad. I've rendered many Greg's accessible extruders and was always irritated that the holes for the motor still had to be drilled through, and the hole in the center where the hobbed bolt goes needed to be awkwardly drilled through the bearings.
Yeah. That's because there are references to a variable called "layer_height" that doesn't exist.
There is a "layer_thickness" variable set to 0.4, but no layer_height. I set it to 0.4 as well, and sure enough, the motor holes now go pretty much all the way through and the hole in the center where the hobbed bolt goes is there.
Rather than upload a derivative that fixes this, I figured I should post that here on the original part. Maybe Greg can upload a v4 within this thing id (I know 8252 by heart) to fix this problem.