# Retrofit Curtain Rail Runners Roller diameter 5.5mm, quiet and lightweight type, compatible with square and C-shaped rails, groove width 6-8mm. 8 pieces with case. I purchased a pack of 12, but only needed 4, so I'm selling the unused ones. There were no instructions, but I was able to set them by inserting them into the rail groove and turning them sideways with a click. They slide well, and I'm using the curtains without any problems. Please consider this with your understanding and agreement.