I designed this spool support to re-use parts left over from converting my PrintrBot Simple from V1 to V2. As printed, the holes will accept the 1/4" threaded rod from the PrintrBot Simple V1. (I use that rod as one of my verticals.) With a little reaming, it will accept the 8mm smooth rods. You might want to be very careful reaming, and not force the rod in, or you might split apart your layers. I'm finding it's simpler not to use a flanged cap in the spool - the spool can ride on the 8mm rod just fine without any kind of interior support or bearing.