This is an MGN12 and 3 point bed leveling conversion for the Ender 3 Pro, it should also work on the OG ender 3 and possibly other printers that use a single 2040 or 4040 extrusion for the Y axis. I have included iges, step and fusion files along with the STL if you want to edit or machine your own. This can also be used as a template to drill the necessary holes by hand. The bed mounting holes are all spaced to match the stock Ender 3 hole spacing.
My goal with this build was to retain as much of the stock look and parts of the printer, and run the belt path as close to center as I could. Since the Ender 3 pro uses 4040 the MGN12 is offset to run on the right side of the extrusion while the belt runs on the left side side.
The front nylon spacers around the front tension are going to need modified. Either cut one down to 5mm and place the remaining piece on the opposite side of the idler or get 25mm and 5mm M4 spacers.
The Y axis motor mount needs to be moved left (as viewed from the front) and 1 hole on the extrusion may need tapped for M5 to attach the 2 bolts. The 3rd bolt bolt can be attached to the extrusion using a 90* corner bracket, 1 M5 T-nut and and M5 SHCS.
My next step is to have this machined in aluminum and take the remaining install pictures.
I modified the slot for the belt, the original version had the slot backwards.
4x M3x10mm SHCS
3x M4x40mm SHCS if you to want to retain springs
3x Bed springs
12x M3x8mm SHCS
12x M3 T-Nut
350mm Linear rail
1x 90* angle bracket
1x M5 T-nut (optional)
1x M5x8mm SHCS (optional)
1x M5 nut (optional)
M5 tap (optional)