The stock extruder on the One/Two up has a bearing arm that is made of very thin MDF board and is one of the things that usually break first on this design. This is a replacement. The smaller holes/slot may need to be drilled out and clean up with a needle nose file to make sure that they can move. I have two versions here, the first one uses the stock bearing. removed for rework [The -02 version uses one of the large bearings (R62RS) from the kit-I have spares of this size. I like this one better than the original bearing because it is wider and has less risk of the filament jumping off track.]