Now that 622 has a motor, I hooked up some alligator clips to see how it behaves. There is a hesitancy at the quarters, which makes me believe there is a slight quartering issue, despite my efforts to limit it. It’s unsurprising because I could feel some stiffness when rolling the chassis by hand.
Really the only adjustment available at this point is to ease out the holes in the coupling rods. How much to ease them is one of those things that can only be learned by experience, unfortunately, and compared to steam modellers who start with RTR models or even kits, I have relatively little. Knowing that it will be difficult to add material back, I made the holes in the coupling rods only big enough to allow their crankpins to turn.
Before reaming any holes, though, I checked how the special hat-shaped washers compared to their related crankpins. I found two that were indeed as much as .07 mm (.003”) wider than their related crankpins, meaning they will have a tighter fit on the coupling rod, and be prone to unscrewing the crankpin bolt (I had indeed encountered this with one of them already).
I made two new washers to better match those rear crank pins. The process was to centre-drill some 1/8″ rod, turn the hat profile, and part it off with a razor saw. This left the washers a little too thick, so I dropped them into a hole in a piece of scrap so I could hold onto them whilst filing them to the correct thickness.
With the new washers, the chassis rolls with less hesitancy. We’re almost there.