This is an adapter for a 6mm tube fitting to a 4mm tube. The fitting screws on the superior part of the adapter fixing the 6mm tube and a M4 nut inside the adapter screws on the 4mm tube fixing it. Remember to add support everywhere when printing, otherwise you will have to file it.UPDATE (05-NOV-2015): Added a 13mm thread adapter version. Now there are two versions, 13mm and the old 9mm. I had the to modify it to 13mm thread because I finished my stock of 9mm thread adapters and got a bunch of 13mm ones. The adapter is the same except for the thread diameter.