This printed great and seems to grab the chain pretty well. I have once comment (potential correction). There is a nut parameter listed as:
"nut_diameter = 5.7; // correct?"
When I printed, the nut cavity measures ~4.8mm wide. The distance between flats on an M3 nut is ~5.4mm. I printed this other pully before : http://www.thingiverse.com/thing:16627. It also has 5.7mm in the code, and I get a measurement of 5.6 when printed (a little shrinkage). Since I am getting 4.8mm on this one with the same printer, material and settings, I think there might be a calculation problem for the width of the M3 cavity. I just fudged it to 6.5mm and then it came out fine. Thanks much.