Update 31-08-16 the machine is much quieter with these mounts the motors are much better isolated from the frame acoustically,which is a nice unexpected improvement.
Okay so there are several issues with the SmartRapCore Alu:
1) as with all corexy machines getting the machine to print correct sized parts in x relative to y is done by adjusting the belt tension from the A belt relative to the B belt. This is difficult on the SCALU machine as the only way to do it is to pull belts out of belt clips and move them one belt tooth (2mm) one way or the other. I tried to get around this by making a balanced belt tension carriage but it still doesn't work perfectly.
2) The motor mounts also being the y-rod holders at the front are not great. I have never been happy my y-rods were perfectly aligned or held firmly enough to ensure good print quality. Plus the left and right sides had different mountings, this never seems like a good idea.
3) The big one - belt angles, this thread http://forums.reprap.org/read.php?344,563301,703546#msg-703546 on reprap forum makes clear the reasons why the belts must be parallel to one another along the x direction, and must turn at exactly 90 deg from the y slides to the back idler pulley and front drive pulley. Any deviation from this and objects will not be square, especially at the edges of the bed. In the original SCALU design this might have been correct but with 608 bearings the geometry changes, and the pulleys are not perfectly aligned with the y-carriages.
So these motor mounts are designed to solve all three problems.
1) The x and y (a and b) motors can now slide around 10mm, to easily adjust the relative belt tension.
2) They are derived from the rear pulley holders, my version which has fully supported pulleys (http://www.thingiverse.com/thing:1655402), so they hold the y-rods firmly and precisely in alignment with the rear pulley holders, no more odd asymmetrical fixings as found in the original SCALU design.
3) The pulleys are now positioned so that the belt turns exactly 90 degrees from the y-carriages to the drive pulley. Please be aware my machine has f608zz bearings and not printed pulley sleeves so the geometry for the pulleys is based around that. I have no intention of doing a version for printed pulley sleeves as they are not accurate and 608 bearings are cheap and cheerful. If you replace your pulley sleeves consider printing y-slides which are designed for them with more relief for the belts, or the belts will rub, also you'll need to change your x carriage as the belt clips are more spaced out for pulley sleeves than for 608 bearings.
Made in Tinkercad, and I am bad at CAD so if anyone who is much better than I am wishes to neaten them up and fillet them etc... then please do. :)
That being said they are perfectly functional.
-I printed mine in PETG, they are very strong, they would work in ABS but PLA might be asking a lot, there are big forces on these parts and it will crack, maybe the reinforced material or annealed versions might work.
-Your current rods should fit okay. On the rear idler pulley holders the large m8 bolt clamps the y-rod, I have changed this to an m3 bolt and locknut with some washers to spread the force out, make sure these are tightened after installation to keep the rods held firmly.
-You can use the four m4 bolts and four m4 half drop-in t nuts (2 per side) from your old motor mounts, but you will also need 2x60mm m4 bolt (57mm to be precise but I used some washers), and 2xm4 drop in t nuts.
-I have not designed an endstop mount, but as you can see from the images I placed one by drilling a 2.75mm hole and screwing the endstop in here. I added a small printed cube bonded with superglue to the y-slide to ensure it can hit the endstop. Future versions will have the endstop hole here, and my version of the y-slides will have an extension for the endstop.
-Attach the motors before installing the units in your machine (I learnt the hard way its easier beforehand - trust me). Use the most accessible three of the holes for your bolts, this will be strong enough, and when you want to adjust the belt tension to calibrate the A and B sides of a cube its easier to undo 3 accessible bolts without the 4th inaccessible one making the job hard. I used good ol' fashioned m3 hex bolts not cap headed machine screws. The same problem as trying to get an allen key/hex driver into a kossel's corners applies here, use normal bolts and a 5.5mm spanner. If you have a 5.5mm ratchet spanner use that, considering buying one.
-In firmware REVERSE YOUR X AND Y STEPPER DIRECTION, in Marlin they should read false under #define INVERT_X_DIR and #define INVERT_Y_DIR as the motors are now upside down. Your y-dimension might change by 5mm or so, change these settings also.
Please excuse the PETG prints the quality is not what I usually produce, they're dimensionally accurate but the surface finish needs some tweaking.
Please do mess with these designs I have made the files public in tinkercad but let me know if you improve on it: