These are the rear pulley holders for a smartrapcore alu printer. On the suggestion of some contributors to Reprap forum, I have remixed these originally from the parts by Smartfriendz but incorporating my strengthening modifications to prevent splitting of the original parts under belt tension. Now a fully supported pulley, this prevents twisting under belt tension by adding to the rigidity to the pulleys. It should prolong the life of this part also, which is under constant belt tension and the increased force when the belt moves.
The version of this printer I am using now has f608zz bearings instead of printed pulley sleeves, but they should still fit. You can use your original m8 bolt.
I've added two parts labelled experimental, if anyone cares to print them and try them and let me know how they get on I'd be grateful. The only change is I've added lugs to slot into the 2020 frame on the sides also, which should be fairly tight fitting, so file them a bit if needed, the idea being it might also help prevent twisting of the part under belt tension.
Please note a fellow reprap forum contributor has come up with an improved version of this type of design http://www.thingiverse.com/thing:1678368 which offsets the pulleys. This seems very promising.
Just print them strong ABS or PET/G works fine.