This is a derivative of two extruders, the original YRUDS MK7 and the derivative of the YRUDS for a 3DR bowden in 3mm. I wanted one that used an MK7 hobbed gear and also would work with 1.75mm filament in a bowden configuration. I also added a mount for a 40mm fan to cool the motor just in case as I was printing this in PLA. None existed that I could see so I made this one. It should mount easily on most any extrusion but, if not I'm sure you could get creative for a mount.I printed it at .25mm layers which is just right because of the gap I purposely put in the compression fit small pulley that allows the small piece to break away after printing. The gap is .50mm so it doesn't print in that spot for 2 layers and makes it just weak enough that it breaks away easily when the print is finished. You will need: 1 x MK7 hobbed gear with 5mm hole(I think they are all 5mm better to be sure) 3 x 625zz bearings 1 x 5mm hex head bolt 40mm in length(for mounting the large pulley) 1 x 5mm socket cap or hex head 20mm in length(for mounting the idler bearing) 1 x 5mm socket cap or hex head 30mm in length(for compression of idler bearing) 1 x 5mm washer(spacer between idler and extruder body) 5 x 5mm nut 1 x HTD 195-3M belt 2 x 3mm cap head screws 12mm in length(for small pulley) 2 x 3mm nuts(for small pulley) 3 x 3mm cap head screws 8mm in length(for motor mounting) 3 x 3mm washers(for motor mounting) 1 x push fit 4mm tube adapter with 1/8" BPST pipe thread(really anything that will fit the hole on the extruder body would work as long as it will accommodate a 4mm tube. I used this one from McMaster-Carr http://www.mcmaster.com/#5508k121/=srnmf8) 1 x NEMA 17 motor optional: 1 x 40mm fan 2 x 3mm cap head screws 12mm in length 2 x 3mm nut You can't see it in the pictures but, you will want to drop in two 5mm nuts into the extruder body before putting the rest of it together. One is for screw that holds the bearing that compresses the filament and the other is for the opposite side of the screw that actually works to compress the filament into the MK7 gear. If you look at the first photo I uploaded you can see I put the 5mm x 40mm in length hex head bolt through the large 56 tooth pulley and used one of the 5mm nuts on the other side to secure the bolt in place and also act as a spacer. Two of the 625zz bearings go on either side of the extruder in the holes designated for them. Rotate the STL file and you will see where they go. After putting those in you put the bolt with the 56 tooth pulley through the two bearings and place the MK7 hobbed gear upside down on this. Use two of the 5mm nuts to secure it all in place(I only have one in the picture I took but, will add a second) and make sure it isn't so tight that is restricts the bearings movement but, tight enough that the MK7 gear can't slip during operation. Also use the set screw on the MK7 for added insurance against slippage. The only other thing to be aware of is to use the 5mm washer under the 625zz idler bearing that compresses into the filament so that it doesn't rub against the extruder body. I guess you could also print out a small spacer if you didn't want to use a metal washer. Also in the picture you will see that I have a 5mm nut on top of the idler bearing that compresses the filament. I only used that because I didn't have a 5mm x 20mm in length screw and had to use a 25mm long screw. If I didn't use the nut then the screw would have hit the 56 tooth pulley. Everything else should be hopefully obvious as to where it should go but, if not leave me a question in the comments and I'll try to get to it.