Rain is finally a good excuse to do some bike work, like replace a worn out middle and granny gear. Not only does a new chain not work with those two cogsets, with an old chain it sounds horrible. Constant grinding sound.
Putting on a new 20 tooth granny from www.mountain-goat.de requires a little filing. At least the instructions recommend using spacers, and as a last resort filing. But looking how it all fits together, spacers seem like a bad idea, require additional derailleur adjustment and testing, and filing seems the quickest way to make this work. It appears the aluminum is pretty soft and quick to file away.
Long term test details to come.