I have always been printing 3mm filament, but heard that people were getting better results with 1.75mm, so I thought I'd give it a try.
My J-head is "convertible" so it's supposed to workk with both 1.75 and 3mm filament. The instructions says to split the conversion tube (a PTFE tube, ID 1.75, OD 3mm) into 4 and put the pieces under the set-screw used to hold the inner workings of the hotend together.
I stole these images from reprap.me, so you can see what I mean:
This method didn't work for me, the tube would be pushed up through the hole in the set-screw. It worked (it only came up a few mm) but it was very hard to get the filament to hit the small hole, without the setscrew guiding it.
So I had to come up with another idea. This is my solution:
Screw a m2 nut onto the PTFE conversion tube (it's a bit hard, but possible)
Trim the tube to be flush with the nut.
Put the set-screw on top to hold everything in place.
Volia, problem solved. This has worked perfectly for me, and loading filament is very easy, as the set-screw is tapered in the end, so it guides the filament into the PTFE tube.