4Pcs Flange Coupling Connector, Rigid Guide Model Coupler Accessory, Shaft Axis Fittings for DIY RC Model Motors (3.17mm)

I use these from an electric motor to a gear on my model fairground rides.Fit the motor axle nicely and are concentric. Holes for bolting through.

Good product, overpriced a little bit

... as described and great fit with my Polulu motors

Good value for money. The only problem was that the very small grub screws came out during transit, and then came out of the plastic bag that contained the flags. Difficult to subsequently find.

Good quality with grub screws to tighten onto a small drive shaft, I used these to attach a pulley wheel to a small 12v motor and they worked really well. not very big so check your sizing before you buy.

Received the 4x couplings and an Allen key - But only one grub screw though and so not 5 stars.I hope I have some laying around...

Grate

These were very robust and good quality items, just need to make sure the hole pattern matches your requirement