After switching to a GT2 belt configuration, and needing a solid belt clamping mechanism without the complications of integrated tensioning, I decided to design my own XY blocks. The factory bronze bushings are very prone to clamping deformation and bind, so I wanted something with separate bushing and belt clamping. I designed these around some replacement bushings I got from Robotdigg that have a 12mm OD instead of the stock 11mm. I made allowances to insert M3 nuts into the blocks for all connections, so no plastic threads are relied on. The one exception being that I left holes in the block back face to thread in an M3 bolt in various positions to trip the limit switches. You decided which need a bolt, and where, for limit switch triggering. Mock it up first, because you won't be able to install those bolts once the block and perimeter shafts are installed. One of the biggest benefits of this design is the ability to square the print head by loosening a single bolt on each block to release the belt. This means no more fumbling with pulley set screws! The other benefit of this is that once the belt is released, there is almost no drag compared to loosening pulleys where you are still moving belts and spinning pulleys on the 8mm shafts. This gives you a MUCH more clear picture of how freely your print head moves. I actually discovered that my printed print head was holding the 6mm shafts slightly out of square, so forcing them square to the 8mm shafts actually induced bind! You can simply loosen the belt clamps on two opposing XY blocks, move the print head back and forth by hand and reclamp.