CNCTOPBAOS3Pcs 8x10mm Shaft Coupling Servo/Stepper Motor Jaw Shaft Coupler 8mm to 10mm Aluminum Jaw Spider Plum Coupling D25L30 Connector for 3D Printer/DIY Encoder/CNC Machine

First thing I noticed when I pulled them out of the box was that the manufacturer didn't every clear the tap shavings out of the threads or off the part so I doubt there is a protective coat on it. The main issue is that they don't really hold together and the part does not have a shaft block to prevent the shafts from contacting and wearing down. That red section should be a full cross section of the part like every other brand has.

For a 3d printer or some other low torque application this would probably be ok. You can not tighten these enough to get a good clamp. The threads just strip right out. I ordered a package of three and broke the threads out with almost no force on two of them. I had a 6-32 tap so I was able to tap the threads a little bigger and that is holding for now but I don’t trust them for my application (CNC router). Save yourself some time and money and get yourself some higher quality ones.

Doesn't clamp tight at all before stripping. There is only one clamping screw on each end, no set screws.

Good fit and sturdy.