This upgrade stiffens up the y-axis alot, and makes levelling the bed MUCH easier, due to there only being one rail. The original 'frog' had a cut-out where the holes need drilling, so only two holes can be drilled. A take-up piece is used to give the penny washers something to pull up against. This thing will allow 30mm more y-axis travel due to the carriage only being 61mm long. The old limits were the bearing housings, which, mine were 101.5mm end to end. The new limits are the belt clamps, drill these further in and you could get another 20-30mm, allowing for a larger, squarer bed. This also gives a few mm's of extra z-height, as the linear carriage and rail sit lower than the old bearings. UPDATE: I've uploaded a jig to allow the carriage to be mounted in the middle of the frog. The jig fits into the side of the frog with the single bearing. The motor needs to be moved to the right side of the printer, and turned around (this means that the #define INVERT_Y_DIR needs to be changed to true.) The belt bearings need to be mounted on the other side of the motor mount.Use the jig to drill two new holes in the frog, the carriage has four holes, 26mm square. Affix carriage to frog using four M4x10 hex bolts (and penny washers) and the take-up piece. Rip out (easiest to lever using a terminal screwdriver) the support pieces out of the hex holes, and fit nylock nuts, these will be tight hopefully, and will need squeezing or pulling in with a M4x16mm bolt. Fit the mounts to the frame/raceway using M4x20mm cap head bolts. The raceway should be approximately 50mm from the right z-axis mount plate.