First of all, the bearings in these are total garbage. They wouldn't slide smoothly at all on a hardened steel shaft. So I swapped them out for some nicer bearings, and all was well.

Second, the mounting holes are not threaded their entire depth, just the first couple millimeters. The threading also wasn't straight on one of mine, but off at about a 15 degree angle. If you need these threaded the entire depth, you need to buy an M4 tap to go with it. Just be careful you don't follow crooked threads and break your tap.

Finally, these still have a rough cast finish. The bearing surface really needs to be milled flat, which I did on a little desktop milling machine.

After my modifications these were decent, but as they come out of the box there are unusable. Don't buy these unless you plan on fixing a lot of flaws.
First of all, the bearings in these are total garbage. They wouldn't slide smoothly at all on a hardened steel shaft. So I swapped them out for some nicer bearings, and all was well. Second, the mounting holes are not threaded their entire depth, just the first couple millimeters. The threading also wasn't straight on one of mine, but off at about a 15 degree angle. If you need these threaded the entire depth, you need to buy an M4 tap to go with it. Just be careful you don't follow crooked threads and break your tap. Finally, these still have a rough cast finish. The bearing surface really needs to be milled flat, which I did on a little desktop milling machine. After my modifications these were decent, but as they come out of the box there are unusable. Don't buy these unless you plan on fixing a lot of flaws.