this is a completed project. It is running successfully on my own printer and does not need any further refinement.
This is the system which I designed for EASY bed leveling for the massive and difficult FT-6 bed. It ditches the tiny knurled aluminum knobs, tiny M3 bolts and short springs and replaces them with M4 hardware, 20mm springs and large, labeled knobs. I found a need for this after leveling for the first time when I realized that there wasn't even enough spring travel to compensate for the 2mm of front-back warpage in my under-bed. I had to use washers under the rear springs. On top of that, the tiny knobs are absolutely awful for making adjustments and the fact that there are eight adds to the frustrations. There had to be a better way.. and now, there is! This setup will require you to buy outside components to get it working but it is very much worth it, especially for making SMALL adjustments on the fly, a task which was impossible to perform with the OE equipment.
The attached .STL file is the bracket which brings all of the other parts together. But you will still need:
2x Ender 3 spring/knob upgrade kits
1x Set of 10 (you only need eight) 5mm (bore) x 10mm (outer diameter) x 4mm (height) ball thrust bearings
1x set of 10 (you only need eight) M4x0.7 x 75mm flat heat stainless steel machine screws
1x set of 20 (you only need 16) M3x0.5 x 10mm stainless steel machine screws
At least 320mm of 6mm ID, 8mm OD tubing
I used carbon fiber:
Alternatively, you could use any stiff material like aluminum, titanium, stainless steel or copper, all of which are available on eBay very cheap
All should come in at less than $40 shipped in the United States. You will have to drill the holes in the bed and under-bed larger to accommodate for M4 hardware. You will also have to countersink the machine screw heads like they original came but for the larger hardware. You can pick up a countersinking bit at any hardware store. Insert the tube into one of the brackets over and over again and trim the excess with a Dremel or mark and cut with a hack saw. Stick the long M4 bolts into the bed and under-bed with the new springs between the two and then slide on the printed brackets with your tube inside. Slide the thrust bearings onto the bolts and then screw on your knobs. The knob will set the upper thrust bearing shell onto the bracket. At this point, you can align the brackets and drill with a 3/32" bit into the ACM and thread on the retaining screws. If the ACMs is too close or too far away, it can be moved. You will have to loosen the M3 hardware holding the under-bed lower braces to the under-bed to get it right. You will lose roughy 20mm of Z travel by doing this mod. If you are running my Y gantry mounts, they were designed to lift Z with this modification in mind and you can reclaim some of that travel. It's a good idea to test out your actual build volume and update your firmware and the printer profiles in Octoprint and your slicer to be on the safe side.
That's all that you need for ultra-smooth bed leveling with LARGE and clearly labeled knobs. This is 100% the best modification that I have donate my FT-6 and I have created a ton of very good mods (all available on my thingiverse) so that says a lot.