The hole leading from the idler/hobgoblin going down into the block is extremely tight. There was a fair amount of stringing on both and the hole is so narrow that 1.75mm filament wouldn't fit through to align the hobgoblin, so I opened it to 2mm with a drill bit. The hole coming out the bottom of the block looks at least 2.5mm, but not sure if the top is supposed to be that large. I have to say without effort at 2mm it is really, really hard to get the filament to drop through. Before assembling it, if I need to widen it further, it would be helpful to know.